How Shift Work May Affect Heart, Metabolism, and Eating Habits

Shift work appeared to be a risk factor for overweight and obesity in a meta-analysis of 28 studies (OR 1.23, 95%CI 1.17–1.29) [41]. Another meta-analysis of 26 studies confirmed this association between shift work and overweight (RR 1.25, 95%CI 1.08–1.44), as well as obesity (RR 1.17, 95%CI 1.12–1.22) [42]. More specifically, this association was more pronounced for abdominal obesity (OR 1.35, 95%CI 1.13–1.61) [41]. However, another meta-analysis of six cross-sectional studies of shift-working nurses did not find an increased prevalence of obesity [43].

While most studies agree on the association between shift work and overweight/obesity, the results are less clear regarding the type of schedules, probably due to the heterogeneity of definitions and schedules patterns. In a Chinese cohort of industrial workers, permanent night workers (i.e., night shifts from 24:00 to 6:00 once a month during >1 year) and irregular night shift workers (i.e., night shifts without a fixed schedule) were at higher risk of overweight (adjusted OR 3.94, 95%CI 1.40–11.03, and OR 1.56, 95%CI 1.13–2.14, respectively) and obesity (adjusted OR 3.34, 95%CI 1.19–9.37, and OR 1.26, 95%CI 0.94–1.69, respectively) compared to rotating shift workers [33].

In their meta-analysis of 26 studies, Liu et al. reported that night shift workers had a higher risk of overweight (RR 1.38, 95%CI 1.06–1.80) than rotating shift workers, similarly for obesity (RR 1.18, 95%CI 1.08–1.29) [42]. These results were consistent with a cohort of Norwegian nurses where night shift workers had an increased body mass index (BMI) compared to day workers [34]. In two large cohorts of American nurses, those working on rotating night shifts (i.e., at least 3 nights per month in addition to days and evenings in the month) were more likely to gain weight over the 18–20 years of follow-up [30].

Furthermore, the duration of exposure, frequency, and intensity of night shift work seems to be a determinant in the development of overweight and obesity, as suggested by Sun et al. [41]. A positive association between the duration of shift work and the prevalence of overweight or obesity has been proposed in several cohort studies [23,30,33]. Ramin et al. reported an association between the exposure to night shifts and the risk of obesity, suggesting that the number of night shifts (from 23:00 to 07:00) per month might be a risk factor for obesity [29]. On the contrary, in the cohort of Norwegian nurses, no association was found between the number of night shifts per year and BMI evolution [34]. The authors suggested a potential survivor effect, i.e., only those tolerating night shift on the long run would keep on this work schedule, or a selection bias in the cohort.

The impact of circadian misalignment on glucose metabolism has mostly been studied in carefully controlled lab studies. In an 8-day crossover study of healthy non-shift workers, Morris et al. showed that circadian misalignment increased postprandial glucose levels by 6% with no difference in fasting glucose levels [12]. The same team found similar results in a 3-day crossover lab study of chronic shift workers [14]. Both studies suggested decreased insulin sensitivity during circadian misalignment. Sharma et al. enrolled rotational shift workers performing a minimum of 3 night shifts per month for at least 1 year, a population similar to the one studied by Morris et al., and found a higher postprandial glucose excursion during the night shift [15]. Bescos et al. reported a decrease in insulin sensitivity after four successive simulated night shifts in healthy adults [16].

While some studies reported no difference in fasting glucose levels [12,14,15], others found an increase of fasting glucose levels [16,56]. These discrepant results may be due to differences in the study design (simulated shift work vs. sleep restriction protocol) or duration of the tested intervention (from days to weeks). Overall, these results suggest that circadian misalignment induced by simulated shift work may contribute to the reduced glucose tolerance [58,59].

Several studies reported an increased risk of T2D in shift workers with a dose–response relationship with the duration of exposure to shift work [30,44,45]. In two large cohorts of American nurses, this increased risk of T2D remained significant after adjustment for BMI [30]. A meta-analysis of 12 cohort studies not only confirmed this association (RR 1.14, 95%CI 1.10–1.19), but also reported a correlation between the exposure to shift work and the risk of T2D in women (per 5-year increase in exposure: RR 1.07, 95%CI 1.04–1.09) [44]. The latter analysis included only female cohorts. In another meta-analysis of 21 cohort and cross-sectional studies, Gao et al. found that shift workers were more likely to develop T2D than day workers (RR 1.10, 95%CI 1.05–1.14), with a higher risk for night and rotating shifts. They also found an association between the number of years of exposure to shift work and T2D (per 5-year increase in exposure: RR 1.05, 95%CI 1.03–1.07) [45].

In a cohort of Danish nurses, Hansen et al. found that evening shifts and night shifts increased the risk of T2D (hazard ratio [HR] 1.29, 95%CI 1.04–1.59 and HR 1.58, 95%CI 1.25–1.99, respectively) compared to day shifts after adjustment for age, BMI, CVD, and lifestyle factors [32]. However, they found no significant risk for rotating shift work.

Finally, Shan et al. found an association between rotating shift and T2D among American nurses [31]. More specifically, the risk of T2D increased with the duration of rotational shift work, i.e., the HR increased from 1.04 (95%CI 1.00–1.09) with 1-5 years of shift work to 1.16 (95%CI 1.09–1.24) after ≥10 years with multiple adjustments including BMI.

Shift work increases the risk of CVD and CVD-related mortality [7,60]. In a meta-analysis of 21 studies, shift workers were more likely to develop CVD events (RR 1.17, 95%CI 1.09–1.25), thus leading to an increased CVD mortality (RR 1.22, 95%CI 1.09–1.37) [46]. More specifically, the risk of coronary heart disease (CHD) (RR 1.26, 95%CI 1.10–1.43) and CHD-related mortality (RR 1.18, 95%CI 1.03–1.32) was higher among shift workers than non-shift workers. The authors reported a dose–response relationship between the duration of shift work and CVD risk, rising significantly after the first 5 years of exposure to shift work (for every additional 5 years: RR 1.07, 95%CI 1.05–1.10).

The effect of circadian misalignment on blood pressure (induced by shift work or simulated) has been studied both in cohorts and in human controlled lab studies with crossover designs. A meta-analysis of 27 studies, including 9 cohorts, reported that shift workers were more likely to have hypertension (OR 1.31, 95%CI 1.07–1.60) [47]. In their 8-day crossover study simulating short-term circadian misalignment in healthy non-shift workers, Morris et al. found increased systolic and diastolic blood pressure after night shifts, and reduced heart rate variability, reflecting a decrease in vagal stimulation [11]. In a related 3-day crossover lab study in chronic shift workers, Morris et al. confirmed the increased 24 h ambulatory blood pressure, in addition to higher high-sensitivity C-reactive protein in circadian misalignment [13].

To explore the relationship of shift work with dyslipidemia, Dutheil et al. reviewed 66 articles and found increased levels of triglycerides (overall standard mean difference [SMD] 0.09, 95%CI 0.05 to 0.13) and decreased levels of high-density lipoprotein cholesterol (SMD −0.08, 95%CI −0.12 to −0.03) among shift workers but no significant difference for low-density lipoprotein (LDL) cholesterol [48]. The changes seemed greater among permanent night workers for total cholesterol and triglycerides. In a cohort of Dutch healthcare workers, Loef et al. found lower levels of total and LDL cholesterol in night shift workers compared to non-shift workers (difference in total cholesterol −0.38 mmol/L, 95%CI −0.73 to −0.04; LDL cholesterol −0.34 mmol/L, 95%CI −0.60 to −0.08) [36]. However, the age, occupation, and education were different between shift workers and non-shift workers, which could partly explain these differences. In another Dutch cohort, Hulsegge et al. did not find any difference in blood pressure and lipids between shift workers and non-shift workers [37]. These discordant effects of shift work on the lipid profile and the confounding factors need to be addressed in future studies.

Several studies have investigated the impact of sleep on eating patterns. Most studies and meta-analyses of healthy adults reported that shorter sleep duration was associated with an increase of the total energy intake [61,62,63]. However, their findings were limited and often divergent regarding the intake of specific macronutrients.

Most studies found an increased intake of fat and decreased intake of protein among short sleepers (≤6 h) [61,64]. Both papers reported an association between short sleep duration, nutritional quality, and irregular eating behavior. In addition, Dashti et al. suggested that short sleepers consumed irregular highly palatable (energy-dense) meals and snacks [61].

In a lab study restricting sleep to 5 h for 5 nights, sleep deprivation increased total energy expenditure by 5% but with a greater increase of total food intake, thus leading to a positive energy balance and a modest weight gain [65]. In their meta-analysis of 11 studies, Al Khatib et al. described the association of sleep deprivation and positive energy balance [62]. They found no effect on energy expenditure, but the impact on weight was not specified.

Interestingly, in a study restricting calorie intake to 10% of energy requirements for 48 h, the duration of deep sleep increased and reverted to baseline after energy intake was restored [66]. This study highlighted a potential bidirectional relationship between sleep duration and energy intake, which needs further study.

While there are several studies on the links between sleep restriction and positive energy balance (Section 4.1), only limited evidence exists on the specific effects of shift work on eating patterns. By extension, changes in the metabolic parameters (Section 3) and eating patterns of shift workers might be mediated by sleep restriction induced by shift work [67]. In addition, potential biases need to be considered while reviewing these studies, such as data collection and dietary assessment, adjustments for potential confounding factors, the heterogeneous designs and studied populations, as well as the sample size [68,69].

Most studies reported no difference in energy intake between shift workers and day workers [17,20,21,49,68]. The comparison of shift schedules (early morning, day, afternoon/evening, or night shifts) showed no difference in energy intake either [50]. Interestingly, Flanagan et al. described no difference in the total energy intake among shift workers during their night shift or non-night shift, but the energy intake was redistributed during the night shift [26]. On the contrary, a few studies found an increased energy intake among shift workers [19,24,29]. For instance, Pepłońska et al. found an adjusted mean energy intake of 2005 kcal (95%CI 1928–2084) among rotating shift workers vs. 1850 kcal (95%CI 1782–1921) among day shift workers (p = 0.007) [24]. However, these studies do not rely on the same methodologies for dietary assessment (e.g., smartphone-based food diary and a wearable device detecting the bite counts [19], food frequency questionnaire [24,29]) and no hard conclusions can be drawn until further studies.

Furthermore, only a few studies investigated macronutrient intake among shift workers and the results were most often discrepant [49]. The systematic review by Cayanan et al. did not find any difference in macronutrient intake during night shift workers compared to non-shift workers [50]. In a comparison of different shifts of American healthcare workers, Chen et al. showed an association between shift work and calorie intake with higher fat and carbohydrate intake [19]. Similarly, among Polish healthcare professionals, daily fat and carbohydrate intake were significantly increased on rotating night shifts compared to day shifts (adjusted mean 78 g/day, 95%CI 74–82, vs. 70 g/day, 95%CI 67–74 for fat; 266 g/day, 95%CI 254–278 vs. 244 g/day, 95%CI 233–254 for carbohydrates) [24].

In a cross-sectional study, Chen et al. compared night and day healthcare shift workers after a series of two to three consecutive work shifts [20]. They found a lower proportion of protein consumption during the meal at the lab in the night shift group but no difference in energy and macronutrients intake between groups. In another lab study of non-shift workers, Cain et al. reported no difference in total calorie, protein, or carbohydrate consumption after a simulated night shift but demonstrated an increased preference for high-fat content foods [17].

While most studies agree that shift work leads to changes in sleeping patterns [19,20,21,29] and that sleep affects eating patterns (Section 4.1), the influence of shift work itself on eating behavior remains elusive [67]. Shift work may impact the food type and content as well as the eating behavior. However, only few studies have assessed the specific effects of shift work.

In their review, Souza et al. found a greater consumption of unhealthy foods, such as saturated fat and soft drinks, among shift workers [51]. In an observational study of Swiss industry workers, Bucher della Torre et al. showed that even one night shift was associated with consumption of unhealthy food (as assessed with the adherence to the French Programme National Nutrition Santé Guideline Score [70]) across a large variety of shift schedules [25]. In a cross-sectional study of Polish rotating shift healthcare workers, Pepłońska et al. specifically identified a greater consumption of energy, fat, carbohydrates, and saccharose as well as a lower consumption of fruits and vegetables [24].

By using a device recording the timestamp of hand movements to the mouth, Chen et al. showed that night shift workers snacked more than day and rotating shift healthcare workers [19]. Similarly, in a cross-sectional study of Canadian nurses, Terada et al. reported an increased consumption of snacks in quantity, frequency, and quality [22].

Finally, in a Dutch cohort, Hulsegge et al. found opposed results, with no difference in meal and snack frequency, nor snack quality between shift workers and day workers [35]. However, snacking habits changed when the workers changed shifts. The contrasted findings may be influenced by several biases as described above (Section 4.2).

Besides the impact of shift work on nutrient intake, the type of consumed food, and eating behavior, shift work might also have an influence on the timing of eating [49,51]. However, this notion is loosely defined in the literature and is very variable between individuals.

Studies have shown that shift work is associated with longer eating duration, i.e., the time interval between the first and the last consumption in a 24 h period [18,21,22]. In an observational study of healthcare workers, those on night shifts self-reported a longer eating duration than day workers (mean 14.2 h ± SD 3.8 h vs. 12.0 h ± 1.5 h, respectively, p = 0.02) [21]. The fasting period was therefore shorter for shift workers (11.8 h ± 2.0 h vs. 13.3 h ± 1.9 h in non-shift workers, p = 0.02) [22]. In a randomized crossover trial, food consumption was nearly around the clock during night shifts compared to day shifts [18].

The energy distribution during the 24-h period can be disrupted by shift work [26,49,51,68,69] without necessarily an increase in energy intake. Multiple meta-analyses were performed and did not always find consistent results in energy distribution between night shifts and day shifts. This is probably due to the lack of adjustment for important covariates, unmeasured confounders, or differences in the shift structure and designs of the underlying studies [49,51,69].

Shift work also leads to irregular meals [22,68,69]. In their meta-analysis, Lowden et al. reported that night shift workers tended to maintain the three meals a day structure, but with less regular patterns especially at night [68]. Moreover, night and rotating shift workers tended to skip meals more frequently [51], and more specifically skip breakfast [23].

These findings on the temporal changes in eating behavior of shift workers explain the increased interest in chrononutrition [21,49,50] and chronotypes (i.e., the individual’s propensity to early sleep and activity, morningness vs. late sleep and activity, eveningness) [71]. The individual chronotype might also influence the tolerance to shift work and its impact on cardio-metabolic health [36,37,72,73], and eating behavior in the general population [74,75,76]. Gupta et al. reviewed the factors influencing the eating behavior of shift workers, highlighting that in addition to food content (the what), the timing of eating (the when), the environment and source of food (the where), and the reasons of eating during the shift (the why) are also part of the eating behavior of shift workers [52].

Yet another avenue could be the central regulation of eating behavior, whereby appetite-regulating hormones could control the hedonic pathways and thus play a role in the changes of eating patterns and in turn the metabolic effects of shift work [27,61,64,77]. However, these aspects are beyond the scope of this review.