The Role of the Chronotype in Developing an Excessive Body Weight and Its Complications—A Narrative Review

The chronotype is an individual’s preference concerning biological and behavioral habits dependent on the circadian rhythm dictated by the oscillations of light levels in the environment during the solar cycle [1]. Researchers created several surveys to determine one’s chronotype, including the Morningness-Eveningness Questionnaire (MEQ), which analyzes a subjective approach to performing activities in a 24 h cycle, the Composite Scale of Morningness (CSM), which puts additional focus on shift work, and the Munich Chronotype Questionnaire (MCTQ), which collects sleep and wake patterns on work and work-free days [2]. Despite the methods described above demonstrating high correlations with each other [3], MEQ is considered to be the best tool to evaluate the differences in peoples’ traits according to the circadian rhythm [4]. Depending on the subject’s body temperature and the intensity of their alertness during the day, the chronotype is divided into three groups: morningness chronotype (MC), intermediate chronotype (IC), and eveningness chronotype (EC) [5]. In the last few years, a topic related to the chronotype has become of great interest to researchers.

It is widely known that the circadian clock determines behavior and lifestyle decisions. It impacts not only people but also other mammals; therefore, it may be concluded that it is a primal form of regulating the metabolism, nutrition, and habits affecting the subjects’ physiology [6]. An awareness that chronotype influences dietary patterns, the sleep cycle, and the level of physical activity resulted in research showing that obesity may be associated with the EC. Moreover, the MC seems to be related to more effective weight loss and adherence to healthier patterns, such as the Mediterranean diet [7,8]. An eveningness preference corresponds to the minor allele of the CLOCK 3111T/C polymorphism, resulting in greater difficulties in weight loss [9]. Subjects with the EC are potentially more predisposed to type 2 diabetes mellitus (T2DM) and higher hemoglobin A1c levels (HbA1c) as a consequence [10]. They are also at a higher risk of other metabolic disturbances, as in people with EC, the level of ghrelin and glucagon-like peptide-1 (GLP-1) is changed. These hormones affect appetite, gastric passage, and insulin secretion; therefore, their level of rotation often results in impaired control over postprandial glucose excursions and increased food intake, leading to weight gain [11].

It has been demonstrated that chronotype categories could have an impact on the risk of developing obesity and obesity-related comorbidities. An excessive body weight predisposes one to low-grade inflammation, which correlates with higher cardiovascular risk factors. This is due to various components, including impaired gut absorption resulting in lower iron levels, myocardial overload leading to an anatomical reconstruction of the heart, endothelial alterations that induce hypertension [12], and a higher possibility of developing T2DM. The EC is less likely to maintain a healthy lifestyle. Therefore, people with this chronotype experience numerous complications due to previously mentioned changes. This may result in heart failure, myocardial infarction, coronary artery disease, angina pectoris, and atrial fibrillation [13]. Furthermore, excess adipose tissue is associated with hormonal impairment and cellular damage, causing female infertility, which is why the MC is proven to have a higher chance of reproducing [14]. Obese individuals tend to undergo severe infections and be less responsive to vaccines due to their weakened immune system [15]. Finally, the EC is correlated not only with a greater predisposition to weight gain but also with more difficulties in maintaining weight loss [7].

Given the rising prevalence of overweight and obesity in many countries [16], this review aims to describe a relationship between a chronotype and the development of excessive body weight and its subsequent complications. The manuscript represents a comprehensive overview conducted between August and November 2024 using the National Library of Medicine browser (Medline, PubMed, Web of Science). The following keywords were used: chronotype, circadian rhythm, obesity, eating behavior, physical activity, hypertension, metabolic syndrome, diabetes, sleep apnea, and liver diseases (Supplementary Materials: search terms). Thus, this work provides a narrative review and discussion of recent data to summarize studies on the circadian rhythm in the context of obesity. The structure of the article is as follows. First, the definition and the determinants of the chronotype are described. Secondly, eating behaviors in the context of the chronotype are reviewed. In the next section, the significance of physical activity is revised. In the last section, the relationship between the chronotype and obesity complications is discussed. Finally, the conclusion with a summary of the current state of research is given.

The chronotype has been defined as an individual characteristic associated with optimal functioning during the day or at night. Although people are classified as a diurnal species due to individual preferences, habits, and predispositions, they can be further divided into three main types of chronotypes, as previously mentioned [17].

The main difference among these groups concerns the sleeping pattern. Subjects with the MC, also called “lark”, prefer to fall asleep and rise early to carry out activities in the first part of the day. On the contrary, people characterized by the EC, called “owls”, usually wake up later and tend to schedule their activities for the late afternoon or evening. While people with the morning chronotype take less time to feel fully alert upon awakening, the evening chronotype reaches its productivity peak in the second half of the day [18] (Figure 1). Nevertheless, most people (about 60%) are classified as the IC, showing characteristics between the morning and evening types. Therefore, they usually present greater flexibility in adapting to daily life without any particular period of time [17]. Moreover, a person identified as the MC or the EC may exhibit its traits to varying degrees (extreme, moderate, or low) [19].

It has been suggested that individual chronotypes also vary in the levels of classic markers for measuring a mammal’s circadian rhythm, including melatonin secretion, the minimum core body temperature, and plasma cortisol levels (Figure 1). Melatonin is a hormone produced by the pineal gland, with its highest concentration observed at night when its secretion is not suppressed by light. Its synthesis is regulated by the suprachiasmatic nucleus (SCN), which serves as the central circadian clock in mammals. Information about the absence of light from the retina reaches the SCN in the hypothalamus. This information is then transmitted via a neural pathway that releases noradrenaline to pinealocytes, stimulating the synthesis and release of melatonin [20]. Endogenous melatonin secretion starts around 2 h before bedtime with a night peak nearly 30 times higher than daytime values. Its production falls during the second half of the night and is inhibited by light. This hormone acts as an internal rhythm synchronizer, promoting sleep by inducing vasodilation and decreasing the core temperature. For this reason, it is considered the most accurate predictor of sleep beginning. It is worth adding that the onset, acrophases, and offset of melatonin secretion occur approximately 3 h earlier in the MC compared with the EC, with no differences in amplitude [19].

It has been shown that, within the rhythmic profile, the onset of melatonin secretion under dim light conditions (dim light melatonin onset, DLMO) is perceived as the most valuable marker for assessing the circadian pacemaker. Similarly to melatonin secretion, the MC presents earlier higher DLMO values, while the EC exhibits an increase in DLMO values later [21]. Zerbini et al. showed that the season and weekly structure are related to changes in the phase of entrainment and sleep. During the summer, sleep times and the DLMO were about 1 h earlier compared to winter, while sleep duration was shorter. Moreover, the DLMO and sleep times were later on work-free days, and their phase differed in workdays. What is important is that these effects were stronger in the EC [22]. In accordance with these findings, in a meta-analysis from the year 2020, Wei et al. indicated that the melatonin level in night-shift workers was significantly lower than in day workers (SMD = −0.101, 95% CI = −0.179 to −0.022, p = 0.012), which suggests the suppression of melatonin production in this population group [23].

As previously mentioned, people with various chronotypes differ according to body temperature. The results from studies involving the measurement of minimum body temperature evaluated a 2–3-h difference in the positions of the circadian phase between the MC and EC [24,25,26]. The comparison of the Tmin between genders emphasized that it typically occurs roughly thirty minutes later in men than in women. Additionally, temperature amplitudes calculated only in male participants were larger in subjects with more delayed phases due to lower nocturnal temperatures [24].

Cortisol, often described as the “stress hormone”, is a steroid compound produced by the adrenal glands, and its secretion is regulated by the hypothalamic–pituitary–adrenal (HPA) axis. It is needed for controlling various functions, such as the metabolism of glucose, proteins, and lipids, as well as managing blood pressure and modulating the immune response [27]. Chronic stressful situations can predispose one to an imbalance in its homeostasis [28]. Cortisol levels show distinct diurnal variations related to the sleep–wake cycle, peaking shortly after waking up and gradually decreasing to their lowest levels around midnight [29]. Bailey et al. conducted a study involving 19 healthy adults, with 10 identifying as the MC and 9 as the EC. Blood samples were collected every 2 h over 38 h and analyzed for serum cortisol concentrations. The results showed that the cortisol acrophases occurred 55 min earlier in the morningness group and that the eveningness group had a smaller cortisol rhythm amplitude [30]. Moreover, Randler and Schaal analyzed saliva samples from 43 adolescents and 82 young adults upon awakening and 30 min later. Their findings indicated that individuals with the MC exhibited higher cortisol levels immediately after waking up [31]. It seems that the more morning-oriented chronotypes, the higher the level of cortisol secretion is observed [32]. These findings may explain why people with the MC achieve full wakefulness more quickly and can undertake various activities effectively in the morning compared to the EC.

A person’s sleep preferences appear to be determined by environmental and genetic factors. It is estimated that the latter may contribute up to 50% of the variability observed in chronotypes. Several genes, known as circadian clock genes, have been identified as potential candidates for their association with various chronotypes [33].

Clock genes play a crucial role in regulating the circadian rhythm of living organisms through a circadian feedback loop. This loop starts with the expression products of the CLOCK (Circadian Locomotor Output Cycles Kaput Gene) and BMAL1 (Brain and Muscle ARNT-like 1) genes forming a heterodimer BHLH–PAS (Basic Helix-Loop-Helix-Per-Arnt-sim), via their PAS domain and bind to the E-box through the BHLH domain, which initiates the transcription of other genes, such as PER (Period gene) and CRY (Cryptochrome). These proteins accumulate in the cytoplasm, and after reaching an ultimate concentration, they form a complex that translocates to the nucleus. Within the nucleus, they inhibit the activity of CLOCK–BMAL1, consequently suppressing their transcription. The cycle begins anew once PER and CRY are degraded [34]. This time-delay cycle through transcription and subsequently the accumulation/degradation of proteins modifies a negative-feedback homeostat into a self-sustained oscillator (around 24 h) [35]. Other regulatory genes, such as RORs (Retinoic acid Receptor–related Orphan Receptors) and the REV-ERB proteins NR1D1 and NR1D2 (Nuclear Receptor subfamily 1, group D, member 1 or 2), respectively, activate or inhibit the transcription of BMAL1 by competing in binding to the mentioned gene [36].

Several studies evaluated a candidate gene approach to assess the relationship between circadian genes and the chronotype. Results showed that individuals carrying one of the two CLOCK alleles, 3111C, exhibited a notably lower average Horne–Ostberg score, indicating a preference for eveningness [37,38]. In other studies, morningness has been found to be associated with polymorphisms in the clock genes PER1 and PER2 [39,40]. A relationship was also noticed between a length polymorphism in the PER3 gene and the occurrence of a specific type of chronotype. Archer et al. compared the variable number of tandem repeats (VNTR) in the PER3 gene among individuals from two groups with extreme scores on the MEQ scale (approximately 7% of 484 volunteers), as well as a control group of equal size consisting of individuals with intermediate scores. It was found that individuals in the EC group more frequently exhibited four repeats than those in the MC group, where five repeats were more common [41]. Although other researchers confirmed these results [42,43], it should be mentioned that there are studies in which no significant correlation between these specific gene variants and types of chronotypes occurred [44,45]. The obtained statistical data exhibit features of a normal distribution, which suggests that there is not a single gene responsible for determining a person’s chronotype, but it is rather the result of an additive, polygenic effect [33]. For this reason, a more current approach is to perform a genome-wide analysis to identify new genetic loci, as in the study by Jones et al. The researchers conducted the most extensive Genome-Wide Association Study (GWAS) on the self-reported chronotype, involving 697,828 participants. The authors identified 351 loci linked to the chronotype preference, of which 24 had been previously documented in an earlier GWAS, and 327 were newly discovered, i.e., PER1, PER2, PER3, CRY, FBXL3, and ARNTL [46]. Similar results were obtained earlier in other GWASs [47,48]. What is more, Molina-Montes et al. established that genetic variants of some circadian clock genes (variant rs2735611 of the PER1 gene and three CLOCK gene variants—rs12649507, rs3749474, rs4864548) could explain the link between genetic susceptibility to the obesity risk and an individual’s chronotype [49].

The chronotype is a relatively constant feature of a given individual, but different age groups display specific tendencies. Typically, young children lean towards an MC before entering their teenage years. Conversely, in the group of adolescents and young adults, an EC is usually observed [2]. This becomes especially visible among college students transitioning to independent living [50]. Then, over time, people stand out to demonstrate earlier sleeping preferences, and the elderly usually show a renewed tendency to be morning-oriented [51,52].

Gender appears to be a factor influencing a person’s chronotype as well. Most studies have observed a more frequent occurrence of the MC in females compared to the EC in males. This may be due to hormonal differences and societal gender roles [53]. Exposure to light also seems to significantly impact the type of chronotype. It has been proven that people born during a season with reduced sunlight intensity, i.e., during a short photoperiod (autumn, winter), have a greater tendency to show a morning chronotype than those born during a long photoperiod, indirectly confirming an imprinting-like phenomenon conditioned by the photoperiod at birth [54]. Additionally, those who classify themselves as the “MC” tend to have more natural light exposure in the morning, whereas the “EC” is exposed to more artificial light during the evenings and at night [55]. Moreover, after four hours of exposure to blue light, results from an electroencephalogram (EEG) indicated that subjects with the MC had elevated theta and low alpha spectral power in the afternoon in comparison to people exposed to polychromatic white light, suggesting that MC individuals may be more sensitive to light [56].

Eventually, social factors may impact the chronotype as they often interfere with subjects’ sleeping preferences. Since exogenous factors, such as the school schedule or shift work, exist, there is often a misalignment between sleep time and activity between life rhythmicity and the biological clock. This phenomenon has been called “social jetlag” [57]. Evening types are particularly vulnerable to social jetlag, as most activities require people to start their efforts early. Therefore, it is unsurprising that people with later tendencies are likelier to report poorer sleep quality and daytime sleepiness [58]. Conversely, research on nurses working night shifts showed that those with an earlier chronotype had more difficulty adapting to their work schedule and suffered more significant social jetlag [59]. In the short term, this misalignment between social demands and the chronotype may decrease productivity. Still, in the long term, it may hurt overall health and potentially lead to various diseases [60,61].

Ekiz Erim and Sert described an interesting finding. The authors investigated the effect of the Circadian Timing Program (SİZAP) developed for EC subjects with the first degree of obesity on obesity management and sleep quality. SİZAP includes instructions for regulating daylight exposure, sleep, caffeine and food intake, and exercise times to ensure proper hygiene training. The research group had a statistically significant decrease in daytime sleepiness and the anthropometric measurements (p < 0.05). Moreover, the scores of the bodily functions sub-dimension of the quality of life scale and the sleep quality scores were better in the SIZAP group (p < 0.05), showing that this therapy may be an effective target in obesity treatment [62]. Muscogiuri also indicates that the physiological timing of energy intake may have anti-obesity potential [63].

The chronotype influences the time meals are eaten and determines preferences, as well as eating habits. It regulates the intake of specific food groups and the total energy per day. Most people distribute their meals according to their biological rhythm rather than the societal schedule. Morning chronotypes consume more energy early in the day, whereas EC shifts their meals to later hours, closer to the evening [64]. Such a difference in meal timing causes the EC to present a greater tendency to omit breakfast [65,66]. A study involving 1854 adults aged 25–74 years found that the EC, unlike morning chronotypes, eats 4–5% less energy in the morning and, at the same time, up to 6–7% more energy in the evening hours [67]. Also, another study conducted by Lucassen et al. on 114 obese individuals showed higher calorie consumption in the mentioned chronotype. It was found that individuals with an EC exhibit approximately 50% greater calorie intake after 8:00 p.m. compared to those with an earlier chronotype [68]. Skipping breakfast and shifting meals to later parts of the day may have negative health implications. Research shows that such dietary patterns affect the cardiometabolic profile [69]. Skipping breakfast may lead to weight gain, especially in individuals who prefer the evening time. The NHANES study on individuals aged 20–39 demonstrated that breakfast consumption reduces the risk of obesity and overweight [70]. Conversely, a study on men from the United States aged 46–81 found that eating breakfast reduces the risk of gaining weight (up to 23%) [71]. Consuming energy after 8:00 p.m. may increase the body mass index (BMI) [72]. Studies also indicate that late dinner consumption is associated with elevated levels of triglycerides and cholesterol [73]. Moreover, consuming large portions of meals during dinner is linked to poorer glycemic control [74].

Individuals who prefer the evening time exhibit additional unhealthy eating habits, as presented by Teixeira et al. in their systematic reviews [75]. They tend to eat more significant portions and opt for a second one. Additionally, they demonstrate a heightened tendency towards emotional eating [76] and eat less frequently than MC [68]. It has been observed that people with the EC have more irregular meal patterns [67]. In a systemic review and meta-analysis conducted by Rodríguez-Cortés et al., a higher tendency of incorrect eating behaviors was observed in children and adolescents with the EC suffering from overweight/obesity [77]. Research indicates that such a situation may contribute to the development of insulin resistance (IR) or increased risk of metabolic syndrome (MetS) [78].

The evening chronotype type consumes more caffeine and alcohol [79]. The higher consumption of these beverages is likely associated with their stimulating properties [80]. A study on 2854 Thai college students revealed that individuals, particularly those with an EC, who frequently experience sleep deficits, consumed higher amounts of caffeine to improve focus and concentration while studying [81]. Moreover, Bodur et al. have pointed out a correlation between poorer sleep quality in the EC and increased caffeine consumption in coping with social obligations [82]. A meta-analysis of 13 studies showed that EC individuals were 41% more likely to consume alcohol versus other chronotypes (OR = 1.41, 95%; CI = 1.16–1.66; I² = 38.0%) [83].

However, no exact conclusion exists regarding differences in daily energy intake between the MC and EC. Some studies suggest that daily energy intake is the same regardless of the chronotype [67,84]. Others show higher energy intake among ECs, as previously mentioned [85,86,87]. There is also inconsistency regarding the types of foods consumed by different chronotypes. Some studies show no differences in macronutrient intake between chronotypes [66,68]. However, a survey conducted by Toktas et al. on 23 male university students found higher carbohydrate and fat consumption and lower protein intake in the EC [85]. Similarly, a study by Sato-Mito on 3304 Japanese female students aged 18–20 indicated lower protein intake in the evening type [88]. Another study involving 4493 adult participants confirmed higher fat and lower protein intake in this chronotype. However, it was found that there was a lower consumption of carbohydrates in the EC [89]. This is also confirmed by a study on 223 German teenagers, where lower carbohydrate intake was also observed in this chronotype [90]. Conversely, a study conducted by Arslan et al. on 204 healthcare workers showed higher carbohydrate intake among individuals who prefer an evening time [86].

Another difference between chronotypes occurred in vegetable and fruit consumption. In a study on 511 British teenagers, the EC was found to have a higher frequency of consuming unhealthy snacks and a lower intake of vegetables and fruits throughout the day [91]. Similar results were obtained in another study by Baron et al. on 52 participants, which showed that individuals with an EC consume fewer vegetables and fruits, as well as more carbonated beverages and fast foods [72]. However, one scoping review shows that while vegetable consumption is lower in the EC, fruit consumption seems similar in both chronotypes [64].

It appears significant that sleep quality, which is observed to be poorer in the EC, influences dietary choices and total energy intake throughout the day [92]. A study of 3072 Istanbul/Turkey residents found that individuals with poorer sleep quality consumed more protein, fiber, and cholesterol, while those with good sleep quality consumed less total energy and carbohydrates. Additionally, men exhibited lower fat intake [92]. A study carried out by Weiss et al. on 240 teenagers also revealed a link between a shorter sleep duration and higher fat consumption. It was found that individuals who slept less than 8 h consumed more energy from fat food than those who slept more [93]. Furthermore, a meta-analysis identified a relationship between higher energy and macronutrient intake and lower sleep quality [94]. These results indicate that poor sleep quality, associated with the EC, leads to increased energy and macronutrient consumption, especially fats.

As observed, the EC tends to shift their meals to later parts of the day, resulting in lower energy intake in the morning and higher intake in the evening. Additionally, they exhibit a higher frequency of skipping breakfast, irregular meal patterns, and unhealthy eating behaviors. They are also characterized by higher consumption of alcohol and caffeine. However, research is inconclusive regarding overall energy and macronutrient intake throughout the day, but the quality of sleep appears to have a significant impact on these aspects.

The evening chronotype seems to be related to many behaviors that impair human health. This relationship is also noticeable in the context of physical activity (PA) [87,95]. The association between the chronotype and PA level is especially noticeable in the population around the mid-twenties [96]. A low level of PA, which is associated with eveningness, is one of the factors leading to health problems and disorders, including gaining weight and being overweight. PA alleviates the negative results on well-being caused by eveningness [97], while an insufficient level of PA is associated with higher depressive symptoms [98,99,100].

There are many physiological factors influencing physical performance that have daily fluctuations. The ones of significant importance are cortisol and body temperature. Morning cortisol levels stimulate metabolic pathways, leading to skeletal protein turnover. Intense physical exercise raises cortisol, which is associated with decreased sports performance due to its catabolic effect and inhibiting protein synthesis [101,102]. The body temperature increases in the early evening [103]. It positively impacts muscle tissue by modifying components of muscular contractions, like calcium uptake and the activity of myosin ATPase [101]. Thus, what is interesting, considering the influence and daily fluctuations in cortisol and body temperature, is that it can assumed that physical activity undertaken in the second part of the day may be more efficient [102]. Additionally, it has been proven that the work rate of exercises shorter than 50 min is higher in the evening [103].

The chronotype seems to influence the total level of PA during the day [87]. As previously mentioned, individuals with an MC tend to be more physically active than people with an EC [95,96]. This correlation was demonstrated between the chronotype and level of physical activity in all its categories: light, moderate, and muscle strengthening [104]. A study involving 4904 adults aged 25–74 years found that the percentage of people with an EC whose PA level is none or very small (28%) is more than twice higher than in the MC group (12.2%) [99]. The amount of time spent on PA and the frequency of PA also differ between chronotypes. A study on 202 healthy women aged 21–65 confirmed that the MC correlates with a higher frequency of PA. The level of PA in the evening-type group was 2.28 (SD = 1.5) times per week, while in the morning-type group, the score was 2.85 (SD = 1.65) times per week (p = 0.042) [100]. This relationship is not only related to the impartial PA level but also to a worse subjective fitness level among ECs [105]. Thus, a low level of PA impacts sedentary behavior, especially in people who represent an EC [96]. Considering all sedentary activities, including those related to work, television viewing, computer use, vehicular travel, and other contexts, it has been observed that individuals with an EC exhibit a higher sitting index. The study by Wennman et al. showed that 20.5% of individuals with an MC demonstrate a high sitting index, while the score in the EC group for this category was nearly two times higher at 35.9%, Table 1 [99].

In 2020, Merikanto et al. published the results of a study in which they demonstrated that adolescents exhibit a greater tendency towards eveningness, and this shift is reflected by a decline in PA levels, which progressively decrease from early to late adolescence. This reduction in PA participation throughout adolescence may increase the risk of obesity and cardiovascular issues in adulthood [105]. Moreover, in a recent meta-analysis from 2024 conducted by Huang et al., it was shown that the MC is associated with a higher PA level and a lower level of sedentary behavior than the EC [106].

Not only is the amount of time spent on PA diverse in groups of different chronotypes, but the physical performance also has different scores. During the comparison of morning race times, individuals with an MC demonstrate superior athletic performance compared to people with an IC and EC [102]. However, Brown et al. showed that when morning and evening scores of the same group are compared, the difference in scores is more significant for the MC group than for the group of people with an EC or IC [107].

Based on a study conducted by Rae et al. in 2015, most MC-oriented people score better in the morning, and most EC-oriented people score better in the evening. However, these individuals who do not have better scores in the part of the day, dependent on their chronotype, swipe training hours during the day. For example, MCs who score better in the evening have trained in the late hours. It may be concluded that the training time affects the hours of the best scoring despite the chronotype [108]. Another element influencing physical performance is the elapsed time after waking up. Facer-Childs and Brandstaetterin indicated that individuals with the EC have the best physical performance at 11.18 ± 0.93 h after waking up, with people with the IC after 6.54 ± 0.74 h and MC-oriented after 5.60 ± 1.44 h. This means that individuals with the EC need twice as long as those with the MC to be active enough to score at the same performance level, Table 1 [109].

Numerous components of physical performance vary across different chronotypes (Figure 2). Most of these variations suggest that individuals with a more pronounced EC tend to exhibit poorer athletic performance. The first one is the change in power. A study conducted by Lim et al. on 340 elite athletes found that the power drop (%) is bigger in individuals with an EC than with an MC. At the same time, the mean power (measured both in W and in W/kg) and peak power (also both in W and W/kg) in a group of people with the MC are significantly higher than in a group of individuals with the EC [110]. The second is oxygen consumption. The data from the study indicated by Hill et al. indicated that EC-oriented people have higher maximum oxygen consumption in the evening than in the morning. However, this difference is not noticed in the MC group [111]. Another factor is the heart rate recovery time (HR) after PA. Sugawara et al. showed that people with the EC need more time to normalize their HR in the morning than those with the MC at the same time of the day or EC-oriented people in the evening. This may result from slower post-exercise reactivation of the vagus nerve for individuals with the EC in the early morning [112]. When analyzing the subjective rating of perceived exertion (RPE), there has not been any diurnal variation in the RPE, even despite a higher evening work rate [103]. Nevertheless, dissimilarities in the RPE were noted in a study conducted by Rae et al. on a group of 26 swimmers based on the timing of the individuals’ training sessions. Those who engage in morning training experience reduced perceived effort during morning exercise sessions, whereas individuals training in the evening exhibit a lower RPE during evening workouts. This is indirectly related to the chronotype as it affects the time of training. This disparity was noticed only after the warm-up, and no RPE change was detected after the whole training session [108]. A study on 20 cyclists revealed that when examining the direct correlation between the chronotype and RPE, it is noticeable that people with the MC experience a lower RPE after self-paced or submaximal physical activity in the morning, compared to their RPE levels in the evening, despite maintaining the same relative intensity workload. However, this relationship is not observed following maximum-intensity performance [113].

Levels of fatigue and vigor may have an impact on physical performance. Individuals with the MC are less fatigued than people with other types of chronotypes in the first part of the day [102]. Thus, MC-oriented people have lower fatigue and higher vigor in the morning than in the evening (Figure 2). In contrast, people with the IC and EC show the opposite pattern—higher fatigue and lower vigor in the morning compared to the evening, Table 1 [108]. In the already mentioned study, Wennman et al. also showed that individuals with the MC have more liveliness in the morning, while those with the EC have much higher alertness in the afternoon or evening. When comparing morning alertness in MC and EC groups, 49% of the MC describe it as good, while 70% of the EC define it as difficult, which may be the reason for the differences in levels of fatigue and vigor noted before [99].

Not only do the physical aspects of PA vary according to the chronotype, but the chronotype also influences specific psychological characteristics, such as physical activity self-efficacy, and the positive attribution of PA effects. A study involving 726 students from German schools aged 10–17 years noticed that individuals with the MC exhibit higher self-efficacy and a more positive assessment of PA’s effects than people with the EC. As a consequence, it seems that individuals with an EC are more predisposed to discontinuing PA engagement, as they do not perceive the outcomes of their PA efforts as positively as individuals with an MC [95].

There has been a noticeable relationship between the chronotype and the choice of sports disciplines. Based on the human body’s response, sports disciplines can be grouped into skill, power, and endurance. There is also a category of mixed sports, including a combination of at least two groups mentioned before [114,115]. A study on 174 African University game players, with a mean age of 21.65 ± 2.05 y, found that IC players mainly participate in skill, concentration, and mixed sports events, excluding football and athletics, which are more likely to be joined by players with the MC [116]. Moreover, MC-oriented people practice endurance, games, and coordinative sports more often [95].

Circadian rhythms may increase people’s levels of PA. Sempere-Rubio et al. concluded that people with stable circadian rhythms have higher PA levels than those with unstable ones. Thus, despite the type of chronotype, keeping a stable circadian rhythm may increase the level of PA, reducing the risk of being overweight [96]. In conclusion with these remarks, in the CORDIOPREV study, it has been observed that subjects with the EC were less active than the MC (201 vs. 251 min/week; p = 0.01) and represented a more sedentary lifestyle (750 vs. 659 min/week; p < 0.01) (Table 1), which correlated with a higher prevalence of MetS in this population group (OR 1.58 IC 95% [1.10–2.28], p = 0.01) [117].

Obesity is a complex health condition in which a person’s chronotype plays a significant role in its development [9]. Individuals with an EC may be especially predisposed to metabolic disorders and irregularities in sleep patterns, which can contribute to weight gain [91]. Those subjects have been reported to tend to follow unhealthy lifestyles mainly characterized by sedentary behavior and the high intake of unhealthy food.

As previously mentioned, clock genes, such as CLOCK, BMAL1, PER, and CRY, play a key role in regulating the circadian rhythm. The expression of these genes oscillates in a roughly 24 h cycle, which forms the basis of the biorhythm. The protein products of these genes create complex feedback loops that drive the biological rhythm. The expression of these genes involved in lipogenesis and lipolysis influences adipose tissue deposition and breakdown. Disruptions in the circadian rhythm can promote excessive adipose tissue deposition and contribute to the development of obesity [49,121]. One systematic review showed that the rates of the minor allele (C) genes, associated with obesity, and SIRT1-CLOCK genes, related to the resistance against weight loss, are higher in individuals with EC [122].

Insulin, a key hormone responsible for regulating blood glucose levels, exhibits daily fluctuations in its secretion. Individuals with an EC may have poorer insulin sensitivity, especially if they consume meals later in the day [123]. When natural insulin sensitivity is lower, that situation can lead to higher post-meal glucose levels and an increased risk of insulin resistance and T2D [124]. An irregular sleep pattern often accompanies individuals with an EC and can further disrupt the normal daily rhythm of insulin secretion [125]. A lack of adequate sleep or sleeping at inappropriate times can lead to disturbances in insulin secretion and increase the metabolic risk [126]. Research shows that there can be a natural increase in insulin sensitivity throughout the day, which is particularly noticeable in the morning [127]. Individuals with an MC often better utilize this increase in insulin sensitivity by consuming breakfast, which can lead to more stable glucose levels throughout the day [123]. Thus, EC individuals have been associated with a higher Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) values, often due to less adherence to a healthy diet and more unhealthy behaviors and eating patterns [122]. What is more, de Almeida et al., in a meta-analysis of randomized crossover studies, showed that the intake of carbohydrates at night leads to higher glycemic (SMD = 1.30; 95% CI, 1.01 to 1.59; I² = 0%; p < 0.00001), but not insulinemic (SMD = 0.19; 95% CI, −0.10 to 0.49), postprandial values in the evening compared to the morning chronotype [128].

Leptin and ghrelin are hormones that regulate appetite. Leptin signals satiety, while ghrelin stimulates appetite. Their secretion can vary depending on one’s chronotype, affecting dietary preferences and meal timing. Individuals with an EC may have higher levels of ghrelin in the evening, leading to increased appetite and later meal consumption, which can contribute to weight gain [11,129].

Cortisol dysregulation can affect glucose metabolism and increase the risk of metabolic disorders, such as insulin resistance and T2D. Additionally, disrupted cortisol rhythms can lead to increased inflammation and changes in blood pressure regulation, further complicating overall health [130]. High cortisol levels can promote the accumulation of adipose tissue, especially around the abdomen, which is associated with a greater risk of developing cardiovascular diseases and T2D [131].

An irregular sleep pattern can reduce the resting metabolic rate, meaning that the body burns fewer calories while at rest. Research has shown that chronic sleep deprivation can affect the hormonal regulation of metabolism, including hormones, such as cortisol, which can disrupt the normal energy balance [132]. Reduced thermogenesis due to sleep disturbances can also stem from poorer body temperature regulation, which is linked to a decreased ability to burn adipose tissue more efficiently [133]. In agreement with these observations, in one prospective study, it has been shown that the EC predicts more significant weight gain and higher BMI values [134].

A person’s chronotype significantly impacts the circadian rhythm and various metabolic aspects, which can contribute to obesity. The impaired insulin sensitivity, adiponectin, melatonin, clock gene functions, and increased leptin secretion detected in subjects with the EC represent favorable traits for weight gain [129]. Understanding these relationships is crucial in creating effective strategies for preventing and managing overweight and in the broader context of promoting metabolic health [135].

Among the most common complications of obesity in the global population are cardiovascular complications (hypertension, coronary heart disease, heart attack, heart failure, arrhythmias, venous thromboembolic disease, stroke), metabolic complications (metabolic syndrome, prediabetes, type 2 diabetes, atherogenic dyslipidemia, non-alcoholic fatty liver disease, gallstone disease), respiratory system disorders (bronchial asthma, chronic obstructive pulmonary disease, obstructive sleep apnea—OSA), neoplasms (esophageal, colon, rectal, prostate, liver, pancreas cancer), mechanical complications (degenerative joint and spine disease), hormonal disorders (polycystic ovary syndrome), dermatological changes (acne), and urinary system complications (chronic kidney disease, obesity-related glomerulopathies) [129,136,137,138,139,140].

While the complications of obesity mentioned above are well documented in the literature, attempts to link excessive body weight and its complications in the context of the chronotype have been relatively recent and have varying results [141,142]. Due to many obesity-related complications, only selected ones have been discussed in the context of the chronotype in this article.

In conclusion, the chronotype represents an individual’s preference for biological and behavioral habits dependent on the circadian rhythm. It is determined by various factors, including genetic and environmental ones, which influence i.e., meal timing and lifestyle decisions. Current evidence suggests a role of the EC in the development of obesity and obesity-related disorders. The evening chronotype has been correlated with lower sleep quality, poorer adherence to healthy eating habits, lower physical activity levels, and unfavorable metabolic changes, such as insulin resistance, higher cortisol levels, or the dysregulation of melatonin secretion. Based on the above, people with evening preferences have a higher tendency of gaining weight and a lower tendency of maintaining a healthy lifestyle. Furthermore, the complications of being overweight are less severe in the MC. People with the EC more often correspond with obesity complications, such as hypertension, disturbances in carbohydrate metabolism, metabolic syndrome, sleep apnea, or NAFLD. Since the human chronotype seems to be related to excessive body weight, patients’ health and nutritional recommendations should involve the influence of the circadian rhythm and chrononutrition. Also, following the physiological timing of energy intake may represent a novel anti-obesity approach. Nonetheless, more clinical studies with larger cohorts are needed to establish the exact relationship between the chronotype and the development of obesity and its complications.