Probiotics and their effects on sleep quality and mood in people with insomnia

A comprehensive search was conducted across eight databases: Embase, PubMed, Cochrane Library, Web of Science, China National Knowledge Infrastructure (CNKI), Wanfang, China Science and Technology Journal Database (VIP), and Sinomed. Subject terms included “insomnia” and “probiotics,” supplemented by terms from the Medical Subject Headings (MeSH) database. The search was restricted to (Title/Abstract) fields, using the following formula: [(Probiotic OR Probiotics OR Bacillus OR Bacillus Bifida OR Bifidobacteria OR Bifidobacterium OR Candida Robusta OR Clostridium Butyricum OR Enterococcu OR Lactobacill OR Lactobacillus Acidophilus OR Lactobacillus Amylovorus OR Lactic Acid Bacteria OR Natto Bacteria OR Saccharomyces Capensis OR Saccharomyces Cerevisiae OR Saccharomyces Italicus OR Saccharomyces Oviformis OR Saccharomyces Uvarum Var Melibiosus OR S. Cerevisiae OR S Cerevisiae OR Streptococcus Thermophiles OR Yeast) AND (Insomnia OR Sleeplessness OR Sleep)]. The search spanned from database inception to December 31, 2024, with no language or other restrictions. Additionally, to ensure a comprehensive literature retrieval and minimize the risk of missing relevant studies, we manually screened the reference lists of all included studies and relevant reviews.

Inclusion criteria were as follows: (1) Participants: patients diagnosed with insomnia per the ICSD-3 or DSM-5. (2) Intervention and comparison: Control groups received no treatment or conventional treatment only, while experimental groups received probiotic treatment in addition to control group interventions. (3) Outcomes: The primary efficacy outcome was the PSQI score. Secondary efficacy outcomes included sleep latency, total sleep time, sleep efficiency, and the Hamilton Depression Scale (HAMD). Safety outcomes were assessed based on total adverse events. (4) Study design: Randomized controlled trials (RCTs).

Exclusion criteria were as follows: (1) Duplicate publications, in cases of multiple reports using the same data, only the most complete version was included. (2) Abstract-only publications without full text. (3) Studies with insufficient or ambiguous data.

Two researchers (YL and YY) independently screened the literature using NoteExpress 3.0 (Beijing Aegean Sea Joy Technology Co., Ltd.) according to the inclusion and exclusion criteria. Duplicate publications, studies irrelevant to the research topic, and those lacking complete data were sequentially excluded to determine the final set of included studies. Disagreements during screening or selection were resolved by a third researcher (SL).

Two researchers (YL and YY) independently extracted basic information (author names, sample sources, sample size, male proportion, mean age, disease duration, intervention, and treatment duration) and outcome data from each study. A third researcher (SL) cross-checked the extracted data to ensure accuracy.

The risk of bias in included studies was independently evaluated by two researchers (YL and YY) using the Cochrane Risk of Bias Tool in Review Manager 5.3. Assessment covered random sequence generation, allocation concealment, blinding, incomplete outcome data, selective reporting, and other biases, with each item classified as low, unclear, or high risk. A third researcher (SL) reviewed the evaluations to ensure consistency.

Meta-analysis was performed using Review Manager 5.3 software. Continuous variables were analyzed using the mean difference (MD) and 95% confidence interval (CI) as effect measures. Dichotomous variables were analyzed using the relative risk (RR) and 95% CI. A fixed-effects model was applied when heterogeneity was low (I² < 50%), and a random-effects model was used when heterogeneity was high (I² > 50%). Statistical significance was set at P < 0.05. Subgroup analyses of outcomes with significant heterogeneity were conducted based on clinical factors, including participant source, proportion of male patients, age, disease course, intervention, and treatment duration, to identify heterogeneity sources. A leave-one-out sensitivity analysis was performed to further investigate the individual sources of heterogeneity and assess the results robustness. Results were considered robust if the combined effect size remained stable after sequentially excluding each study. Publication bias was evaluated using funnel plots, with symmetrical scatter point distribution indicating no bias. The certainty of evidence for each outcome was assessed using Grading of Recommendations Assessment, Development and Evaluation (GRADE) system, considering risk of bias, inconsistency, indirectness, imprecision, and publication bias. Evidence was graded as high, moderate, low, or very low based on these domains, with downgrading for limitations (e.g., high risk of bias, heterogeneity, or imprecision) and upgrading for factors increasing confidence. This approach ensured transparent and consistent evidence grading.

A total of 1,101 relevant studies were identified through the database search: 97 from PubMed, 135 from Embase, 190 from the Cochrane Library, 234 from Web of Science, 162 from CNKI, 142 from Wanfang, 71 from VIP, and 70 from Sinomed. During screening, 407 studies were excluded due to duplication, and 688 were excluded for not meeting the inclusion criteria. Six studies were ultimately included (Ding et al., 2024; Yan et al., 2024; Yu and Zhan, 2023; Byun et al., 2018; Murakami et al., 2024; West et al., 2020), as shown in Figure 1.

Six clinical studies, published between 2018 and 2024, were included: three from China, one from Japan, one from South Korea, and one from Australia. These studies enrolled 424 patients with insomnia, with a mean proportion of male patients of 29.5% and a mean age of 39.3 years. Of these, 223 patients received probiotic interventions, while 201 received no probiotic treatment, as detailed in Table 1.

The Cochrane Risk of Bias tool was used to evaluate the included studies. Four studies had an unclear risk of bias in random sequence generation due to insufficient reporting of randomization methods. Five studies had an unclear risk of bias in allocation concealment due to unreported methods. Two studies had an unclear risk of bias in participant blinding owing to the absence of a placebo design. All other domains were assessed as low risk. Of the six included studies, four were judged to have an overall low risk of bias, whereas two had an overall high risk of bias, as shown in Figure 2.

Funnel plots for PSQI, sleep efficiency, and sleep latency showed symmetrical scatter distributions, indicating no publication bias. However, funnel plots for total sleep time, HAMD score, and total adverse events displayed asymmetrical distributions, suggesting potential publication bias, as shown in Figure 6.

The GRADE system indicated moderate certainty for PSQI, sleep efficiency, and sleep latency, low certainty for total adverse events, and very low certainty for total sleep time and HAMD scores. However, the overall recommendation strength was strong, as detailed in Table 3.

Insomnia is a widespread condition that significantly impacts human health, particularly among older adults (Milner et al., 2021). Chronic insomnia can impair physical function and mental wellbeing and is associated with severe consequences, including increased risks of suicide and sudden death (Aernout et al., 2021; Roth, 2007). Although the therapeutic potential of probiotics for insomnia has attracted growing interest, high-quality evidence supporting their benefits remains scarce. To the best of our knowledge, this is the first systematic review and meta-analysis evaluating the efficacy of probiotics in the treatment of insomnia. Our findings revealed that the probiotic interventions were associated with a significant reduction in PSQI and HAMD scores compared to controls. However, no significant improvements were observed on total sleep time, sleep efficiency, or sleep latency. Furthermore, the analysis indicated no significant difference in adverse event rates between the probiotic and control groups, suggesting that probiotic interventions are generally safe.

The PSQI, a widely used measure of sleep quality, served as the primary efficacy outcome. Our results demonstrated that probiotics significantly reduced the PSQI score by 2.1 points compared to the placebo or no treatment, suggesting that probiotics effectively improved sleep conditions. Sensitivity analysis confirmed the robustness of these results. Subgroup analysis by participant source revealed significant PSQI reductions in participants from China (MD −4.24, 95% CI −5.16 to −3.33, p < 0.00001, I² = 36%) and Australia (MD −1.30, 95% CI −2.54 to −0.06, p = 0.04, I² = 0%), but not in those from Japan (MD −0.28, 95% CI −1.00 to 0.44, p = 0.45, I² = 0%) or South Korea (MD −0.10, 95% CI −1.94 to 1.74, p = 0.92, I² = 0%). These findings indicate that participant source contributed to PSQI heterogeneity. Variations in insomnia risk factors across racial and regional groups, such as higher risks among African Americans and Caucasians, may explain these differences (Kim et al., 2012; Roth et al., 2011). Thus, the effects of probiotics on sleep may differ across populations in different countries and regions. Furthermore, this study found no significant effects of probiotics on secondary sleep indicators, including total sleep time, sleep efficiency, and sleep latency. However, these non-significant results may be due to the limited sample sizes. Therefore, the impact of probiotics on these additional sleep metrics should be further investigated in future studies.

The mechanism by which probiotics may help treat insomnia likely involve the regulation of neurotransmitters, hormones, and inflammatory responses. From a neurophysiological perspective, the vagus nerve serves as a direct communication pathway between the gut and the brain. Microbial metabolites, such as short-chain fatty acids (SCFAs) can stimulate the vagus nerve and influence neural activity in sleep-related brain regions, including the hypothalamus. For example, Lactobacilli have been shown to enhance GABA synthesis, thereby inhibiting central nervous system excitability and promoting deeper sleep (Li et al., 2024). Fermented milk enriched with Lactobacillus brevis DL1-11 has been reported to elevate GABA levels and improve sleep duration and latency in mice (Yu et al., 2020).

Hormonal regulation also plays a role. Gut microbiota influence the synthesis of serotonin (5-HT) and melatonin—key hormones in sleep regulation. Spore-forming Clostridium species can convert tryptophan into serotonin precursors, indirectly promoting melatonin secretion and circadian rhythm regulation (Gao et al., 2018; Wang et al., 2023). In one study, B. coagulans BC99 improved sleep quality by increasing brain 5-HT levels (Sun et al., 2025). Furthermore, modulation of the HPA axis has been implicated (Levenson et al., 2015). For instance, Bifidobacterium breve CCFM1025 improves sleep quality by regulating serum daidzein levels, affecting HPA axis activity (Lan et al., 2023).

The anti-inflammatory effects of probiotics may further contribute to improvements in sleep quality. Previous studies have shown that disruptions in gut microbiota can trigger inflammation, leading to elevated levels of pro-inflammatory factors such as interleukin (IL)-6 and tumor necrosis factor (TNF-α). This inflammation may impair sleep by activating the microbiota-gut-brain axis (MGBA) (Zheng et al., 2023). Conversely, probiotics can inhibit intestinal inflammation by modulating the balance of gut microbiota and restoring the integrity of the intestinal mucosal barrier (Bermudez-Brito et al., 2012; Cristofori et al., 2021; Ng et al., 2009). For example, the probiotic formulation SLAB51 has been shown to restore gut-brain axis hormone levels and alleviate insomnia-induced inflammation in both peripheral tissues and the brain (Sánchez et al., 2017). Additionally, supplementation with GABA-producing probiotics has been found to reduce the levels of pro-inflammatory cytokines IL-8 and TNF-α (Zhao et al., 2022). In summary, probiotics may improve sleep quality by regulating neurotransmitters, hormones, and inflammatory factors.

Negative emotional states, including depression and anxiety, are not only risk factors for insomnia but may also be exacerbated by it. Data from a Chinese survey revealed a strong associations between insomnia and both depression (OR 11.29, 95% CI 9.58–13.29) and anxiety (OR 10.98, 95% CI 8.78–13.72) (Shan et al., 2024). A longitudinal cohort study further confirmed that insomnia predicts the persistence of anxiety and depression (Meaklim et al., 2023). These findings suggest that negative emotions such as depression and anxiety may create a vicious cycle with insomnia, resulting in progressive deterioration in sleep quality. While the primary focus of this study was the effect of probiotics on sleep quality, their potential influence on negative emotional states—particularly depression and anxiety—was also evaluated. Notably, previous meta-analyses have demonstrated the effectiveness of probiotics in regulating mood disorders. One meta-analysis of 23 randomized controlled trials found that probiotics significantly alleviated symptoms of depression (SMD −0.96, 95% CI −1.31 to −0.61) and anxiety (SMD −0.59, 95% CI −0.98 to −0.19) (Asad et al., 2024). Therefore, our study does not explore the broader effects of probiotics on depression and anxiety in the general population. Instead it focuses specifically on their impact on negative emotions in individuals with insomnia. As the included studies reported only HAMD scores and did not provide anxiety-related data, this meta-analysis assessed the effects of probiotics on depression alone.

Our meta-analysis revealed that, compared to placebo or no treatment, probiotics significantly reduced the HAMD score by 7.72 points. This finding highlights the potential of probiotics in lowering the risk of depression and provides additional insights into their role in improving sleep quality. Previous studies support these findings. Two randomized controlled trials reported that healthy individuals who consumed probiotics (Lactobacillus casei strain Shirota and Lactobacillus gasseri CP2305) were able to maintain satisfactory sleep quality even under depression or anxiety (Takada et al., 2017; Nishida et al., 2017). Another study demonstrated that Bifidobacterium breve CCFM1025 enhanced subjective sleep quality in patients with insomnia and reduced sleep disturbances, accompanied by a more pronounced reduction in stress markers (Lan et al., 2023). Collectively, these results suggest that probiotics exert meaningful effects in alleviating negative emotions such as anxiety and depression, aligning with the broader goal of treating insomnia.

The mechanisms by which probiotics alleviate depression may involve the inhibition of inflammatory responses, the reduction of cortisol levels, and the enhancement of brain-derived neurotrophic factor (BDNF) expression. First, probiotics downregulate pro-inflammatory factors such as TNF-α, IL-1β, and IL-6, thereby reducing microglial overactivation and inflammatory damage in the hippocampus, ultimately improving depressive symptoms (Park et al., 2018; Tian P. et al., 2022). Second, probiotics lower cortisol levels via neuroendocrine pathways, thus reducing the incidence of mood disorder-related symptoms (Tian P. et al., 2022; Qian et al., 2024). Third, probiotics can promote nerve regeneration by increasing BDNF levels, contributing to the alleviation of depressive symptoms (Suda and Matsuda, 2022). These findings elucidate the biological pathways through which probiotics may reduce depression risk and further support their therapeutic potential in the management of insomnia.

Adverse events occurred in 16.5% (15/91) of patients in the probiotic group and 9.3% (7/75) in the control group, with no significant difference between groups. Among the included studies, Byun et al. (2018) reported four adverse events: three in the probiotic group (abdominal discomfort, headache, drowsiness) and one in the control group (drowsiness). Drowsiness was attributed to sleep deprivation, not probiotics or placebo. The abdominal discomfort and headache were mild, suggesting good tolerability. Murakami et al. (2024) conducted a comprehensive safety assessment of probiotics through medical interviews, physical measurements (height and weight), physiological tests (blood pressure and pulse rate), and relevant examinations (hematology, blood biochemistry, and urinalysis). They found no significant differences in the incidence of adverse events between the probiotic and placebo groups. Moreover, changes in clinical test parameters remained within physiological ranges in both groups (Murakami et al., 2024). Additionally, a long-term study involving 156 participants found no significant abnormalities in blood cell counts, biochemical tests, urinalysis, or electrocardiograms (Lee et al., 2021). Although adverse events occurred in 2.6% (2/78) of cases, they were mild, self-limiting, and did not lead to treatment discontinuation (Lee et al., 2021). Collectively, these findings support the conclusion that probiotics are safe and well-tolerated for the treatment of insomnia.

This study provides biologically plausible explanations and clinical evidence supporting the use of probiotics to improve sleep and mood in individuals with insomnia. However, several limitations must be acknowledged. First, the inclusion of only six studies with 424 participants may have reduced result precision and generalizability. Second, variability in probiotic strains, dosages, and intervention durations introduced clinical heterogeneity, but the limited number of studies precluded subgroup analysis or meta-regression to explore these factors. Consequently, optimal strains and dosages remain unclear. Third, five studies lacked reported concealment methods, and two omitted blinding procedures, increasing risks of selection and performance biases. The mean participant age of 39.3 years limits applicability to older adults, who exhibit distinct physiological and insomnia-related characteristics.

Future research should address these limitations. Larger, well-designed randomized controlled trials with diverse populations are required to enhance precision and generalizability. Rigorous reporting of concealment and blinding methods will minimize bias. Systematic investigation of probiotic strains, dosages, and intervention durations is essential to identify optimal regimens. Including older adults will clarify the efficacy and safety of probiotics in this demographic. These efforts will support the development of personalized probiotic therapies for insomnia.