Does vitamin B12 affect sleep?

No trial shows that vitamin B12 helps people sleep. We found no meta-analysis that pools B12 trials for sleep. In the one placebo-controlled trial in a sleep disorder, 3 mg of methylcobalamin a day for 4 weeks did nothing for night sleep in 50 people with delayed sleep phase syndrome. A year of daily B12 in 600 Nepali infants did not change their night sleep either. In a small trial in 20 healthy adults with no placebo group, the same 3 mg dose shortened sleep time.

Unsettled. The answer is unsettled because the evidence is small and points in different directions. A systematic review of 26 studies on micronutrients and sleep found no definitive effect of B12 on night sleep, apart from one trial with shorter sleep. Observational studies link low B12 to insomnia in some groups and high B12 to insomnia in others. We found no trial of B12 for insomnia in adults, or in people with a confirmed deficiency.

What the trials found

The largest trial is the one in infants. In Bhaktapur, Nepal, 600 infants aged 6 to 11 months who were shorter than average for their age, and at risk of deficiency, got B12 or placebo every day for a year. Sleep was measured with activity monitors, a predefined secondary outcome. There was no effect on night sleep, total sleep or waking after sleep onset, and no subgroup differed, including the infants who started with low B12. It says nothing directly about adults.

The adult trial in a sleep disorder recruited 50 people aged 13 to 55 with delayed sleep phase syndrome, 27 on methylcobalamin and 23 on placebo. After 4 weeks of 3 mg a day there were no significant differences in sleep logs, daytime mood or drowsiness. The authors concluded methylcobalamin is not an effective treatment for the condition. It is small, short and relied on self-reported sleep.

Myth. Methylcobalamin is sometimes sold as a fix for a body clock that runs late. The only double-blind trial of it for delayed sleep phase syndrome, in 50 people, found no benefit over placebo at 3 mg a day for 4 weeks.

The two remaining studies are smaller. In 20 healthy adults given 3 mg a day of either methylcobalamin or cyanocobalamin for 14 days, sleep time fell significantly on methylcobalamin, and the authors described an alerting effect. There was no placebo group, so the change is measured against each person's own baseline. In a blinded placebo crossover study of 9 healthy people, 3 mg of methylcobalamin for 4 weeks moved the melatonin rhythm 1.1 hours earlier but did not change when they slept. Its abstract does not say the order was randomized.

The wider reviews add an absence. A 2025 meta-analysis of 28 placebo-controlled trials of supplements for sleep found pooled improvements in sleep quality, with Pittsburgh sleep quality scores 0.70 points better (95% CI 0.03 to 1.37). It credited those gains to tryptophan, vitamin D, omega-3, zinc and antioxidants. It contained no B12 trial at all.

What the observational studies show

Four cross-sectional studies looked at blood B12 and sleep, and they do not agree. In 512 Greek primary care adults, B12 below 342 pg/mL went with 2.43 times the odds of insomnia symptoms (95% CI 1.33 to 4.45), mainly in older, female and non-obese people, and supplement users were excluded. The same study found no significant link with poor sleep quality. In 1,468 people with sleep apnoea tested overnight in a sleep lab, B12 below 380.5 went with 24% higher odds of taking 40 minutes or more to fall asleep (OR 1.24, 95% CI 1.005 to 1.531). In 418 Chinese adults with type 2 diabetes the direction reversed. Higher B12 went with 61% higher odds of insomnia (OR 1.61, 95% CI 1.06 to 2.45), and the high levels were tied to taking mecobalamin, a form of B12 prescribed for nerve damage. In 355 female Saudi students, 72% of them poor sleepers, sleep quality was not directly associated with blood or dietary B12. These are odds from one blood test each, and none of the abstracts gives an absolute rate.

Who should be careful

Not for everyone. High doses of methylcobalamin may shorten sleep rather than lengthen it. That is what the small trial in 20 healthy adults found at 3 mg a day, measured against their own baseline with no placebo. In a 2021 meta-analysis of B12 for autism, 7.6% of children given methylcobalamin injections across 2 placebo-controlled trials had trouble sleeping, alongside irritability in 3.4% and hyperactivity in 11.9%. Those were injected doses in children with autism, and the reviewers judged the injections well tolerated for most.

The diabetes study points the same way. High blood B12 in the people with insomnia was linked to mecobalamin use, which could be the supplement or the nerve damage it treats. Two groups have a real reason to check their B12 rather than guess. According to the US Office of Dietary Supplements, gastric acid inhibitors and metformin may affect B12 levels, so people on these medicines are the ones in whom a true deficiency is plausible. The same agency sets no upper limit for B12 because of its low potential for toxicity. No upper limit is not evidence that a large dose helps sleep.

What expert bodies say

The American Academy of Sleep Medicine 2015 guideline, built on a systematic review graded with GRADE, found insufficient evidence to support oral vitamin B12 for delayed sleep-wake phase disorder and made no recommendation for it. That statement rests on the single 1997 trial in 50 patients described above. We found no expert body that recommends B12 for insomnia or general sleep.

How we searched

What would change this answer

• A randomized placebo-controlled trial of B12 for insomnia in adults, with sleep measured objectively. We found none, and ClinicalTrials.gov lists no active registered trial.
• A trial in people with a confirmed B12 deficiency that measures sleep. If B12 matters for sleep, this is where the effect should show, and we found no such trial.
• A larger placebo-controlled test of whether high-dose methylcobalamin shortens sleep in healthy adults. The signal comes from 20 people with no placebo, and it is the finding most relevant to anyone taking a high-dose product at night.
• Studies that measure B12 before sleep problems start. The current links come from single blood tests and point in opposite directions.

For what B12 does, how much you need and who runs short, see the vitamin B12 guide.

Sources

1. ev-b12s-01 · Observational data · systematic review of experimental and observational studies, no meta-analysis · n = 26 articles
   No significant or definitive effect of vitamin B12 on sleep phase and sleep duration at night was reported in early human studies(24,49,50), except the clinical trial by Mayer et al.(51) that reported an alerting effect of vitamin B12 supplementation with a decrease in sleep duration.
   
   Who: individuals from infants to older adults in experimental and observational studies
   Effect: B12: no significant or definitive effect on sleep phase or night sleep duration; one trial (Mayer 1996) reported an alerting effect with shorter sleep; serum B12 inversely related to sleep duration in one adult study
   Certainty: Searched to January 2016; only a handful of small B12 studies; qualitative synthesis mixing trials and observational data.
   Public Health Nutr, 2017 · checked 2026-10-07 · we read the fulltext
2. ev-b12s-02 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 28 RCTs
   These findings suggest that tryptophan, vitamin D, omega-3, zinc, and antioxidants may enhance sleep quality by decreasing SL, and WASO increases SE and extends TST, respectively.
   
   Who: people with sleep disorders and healthy individuals in randomized controlled trials of dietary supplements
   Effect: all supplements pooled: PSQI MD -0.70 (95% CI -1.37 to -0.03); sleep latency SMD -0.24 (95% CI -0.37 to -0.10); vitamin B12 not among the tested supplements
   Certainty: Quote from the abstract; the local full text (PMC12736316, pmc_search.py --show --sec all) was searched for B12 and cobalamin by n6 on 2026-10-07: no hit; vitamins covered were B6 and D. The absence of B12 is the finding for this page.
   Nutrients, 2025 · checked 2026-10-07 · we read the abstract
3. ev-b12s-03 · Position of an expert body · clinical practice guideline based on systematic review, meta-analysis and GRADE · n = 1 RCT, 50 patients
   There is insufficient evidence to support the use of oral vitamin B12 (and no evidence to support alternate somatic interventions) among patients with DSWPD (versus no treatment).
   
   Who: patients with delayed sleep-wake phase disorder (DSWPD)
   Effect: no clinically significant differences in subjective total sleep time or sleep onset time with 3 mg/day B12 vs placebo for 4 weeks; no recommendation
   Certainty: Position of an expert body (graded E); the B12 statement rests on a single 1997 RCT (9413873); earlier AASM practice parameters had called B12 'not indicated'.
   Journal of Clinical Sleep Medicine, 2015 (AASM clinical practice guideline, Auger et al.) · checked 2026-10-07 · we read the fulltext
4. ev-b12s-04 · Randomized controlled trial(s) · multicentre double-blind placebo-controlled randomized trial, 3 mg/day methylcobalamin, 4 weeks · n = 50
   These results indicate that 3 mg methylcobalamin administered over 4 weeks is not an effective treatment for DSPS.
   
   Who: 50 patients with DSPS aged 13-55 years, 27 on methylcobalamin and 23 on placebo
   Effect: no significant differences in sleep-log night sleep, daytime mood or drowsiness
   Certainty: The only placebo-controlled B12 trial in a sleep disorder; small, subjective outcomes, short.
   Psychiatry Clin Neurosci, 1997 · checked 2026-10-07 · we read the abstract
5. ev-b12s-05 · Randomized controlled trial(s) · individually randomized double-blind placebo-controlled trial, daily B12 for one year, sleep a predefined secondary outcome · n = 600
   There was no effect of vitamin B12 supplementation on sleep duration at night, total sleep duration, or WASO.
   
   Who: infants 6-11 months old in low to middle income neighborhoods of Bhaktapur, Nepal, length-for-age below -1 z-score
   Effect: no effect on night or total sleep duration or wake after sleep onset; a small negative effect on sleep onset latency; no subgroup (stunting, low B12, anemia) differed
   Certainty: Largest B12 trial with objective sleep measurement; infants only. NCT02272842.
   Clin Nutr, 2022 · checked 2026-10-07 · we read the abstract
6. ev-b12s-06 · Observational data · randomized single-blind comparison of cyanocobalamin and methylcobalamin 3 mg/day for 14 days after 9 days of observation, no placebo arm · n = 20
   Sleep time was significantly reduced under MB12 intake.
   
   Who: six women and 14 men, mean age 35-37 years, healthy
   Effect: sleep time significantly reduced with methylcobalamin; urinary melatonin metabolite reduced with both B12 forms; night activity increased
   Certainty: No placebo arm, changes measured against baseline, so graded C despite randomization between two B12 forms; preliminary study of 20 people.
   Neuropsychopharmacology, 1996 · checked 2026-10-07 · we read the abstract
7. ev-b12s-07 · Observational data · blinded placebo-controlled crossover trial, randomization not stated, 3 mg/day oral methylcobalamin for 4 weeks · n = 9
   On the other hand, vitamin B12 did not affect the timing of sleep.
   
   Who: 9 healthy subjects
   Effect: 24-h melatonin rhythm phase-advanced by 1.1 h; lower mean melatonin; no change in sleep timing
   Certainty: Tiny mechanistic study; randomization not described in the abstract, so graded C.
   Experientia, 1992 · checked 2026-10-07 · we read the abstract
8. ev-b12s-08 · Observational data · cross-sectional study · n = 512
   We found that low vitamin B12 levels were associated with a 2.4-times risk increase of insomnia symptoms and the association was significant in the older, female, and non-obese subjects.
   
   Who: 512 consecutive adult primary healthcare users in Greece
   Effect: insomnia symptoms (AIS): OR 2.43 (95% CI 1.33 to 4.45); poor sleep quality (PSQI): OR 1.42 (95% CI 0.68 to 2.96), not significant
   Certainty: Single blood measure, self-reported sleep; the authors write 'risk' for an odds ratio. Supplement users were excluded.
   Healthcare (Basel), 2023 · checked 2026-10-07 · we read the fulltext
9. ev-b12s-09 · Observational data · cross-sectional study with type-1 polysomnography · n = 1468
   After adjustments, Vitamin B12 levels < 380.5 were associated with 24% higher odds of prolonged sleep latency (≥40 min) prevalence (OR = 1.240, 95% CI = 1.005-1.531, p = 0.045)
   
   Who: patients with OSA from the Sleep Disorders Center, University of Crete
   Effect: sleep latency >=40 min: OR 1.24 (95% CI 1.005 to 1.531); NREM >80%: OR 2.31; REM <20%: OR 2.86
   Certainty: Objective sleep data but a clinic population with sleep apnoea; association only.
   Nutrients, 2025 · checked 2026-10-07 · we read the abstract
10. ev-b12s-10 · Observational data · cross-sectional study · n = 418
    Moreover, increased VitB12 levels in patients with insomnia were closely correlated with the use of mecobalamin.
    
    Who: 418 patients with type 2 diabetes, China, 2018-2020
    Effect: higher B12 and insomnia: OR 1.61 (95% CI 1.06 to 2.45); cut-off 517.5 pg/mL
    Certainty: Opposite direction to the studies of low B12; reverse causation and confounding by neuropathy treatment are possible. Fits the alerting effect in 8914118.
    Nutr Diabetes, 2022 · checked 2026-10-07 · we read the abstract
11. ev-b12s-11 · Observational data · cross-sectional study · n = 355
    However, sleep was not found to be directly associated with either serum vitamin B12 or dietary vitamin B12.
    
    Who: female college students at King Saud University, mean age 20.7 years
    Effect: no direct association of PSQI with serum or dietary B12; better scores only within upper B12 tertiles in subgroup analysis
    Certainty: Subgroup signal only; 72% of participants were poor sleepers.
    Int J Environ Res Public Health, 2021 · checked 2026-10-07 · we read the abstract
12. ev-b12s-12 · Position of an expert body · agency fact sheet · n = —
    The FNB did not establish a UL for vitamin B12 because of its low potential for toxicity [1].
    
    Who: general population
    Effect: no UL set by the Food and Nutrition Board
    Certainty: No UL does not mean high doses help sleep; the methylcobalamin trials point to shorter sleep.
    NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals · checked 2026-10-07 · we read the section
13. ev-b12s-13 · Position of an expert body · agency fact sheet · n = —
    Vitamin B12 supplements may interact with medications, and some medications, including gastric acid inhibitors and metformin, may affect vitamin B12 levels.
    
    Who: individuals taking gastric acid inhibitors or metformin
    Effect: reduced B12 absorption and serum levels
    Certainty: People on these drugs are the ones in whom a true deficiency, and its symptoms, is plausible.
    NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals · checked 2026-10-07 · we read the section
14. ev-b12s-14 · Meta-analysis or systematic review · systematic review of 17 studies with meta-analysis of adverse events from 2 double-blind placebo-controlled injection trials · n = 2 studies
    There was a low incidence (<5%) of most AEs for subcutaneously injected mB12 with only three AEs reaching significance: increased irritability (3.4%), trouble sleeping (7.6%) and increased hyperactivity (11.9%), suggesting that for most individuals with ASD, subcutaneously injected mB12 is well tolerated without AEs.
    
    Who: children with autism spectrum disorder treated with subcutaneous methylcobalamin, 64.5-75 ug/kg/dose
    Effect: adverse events with injected mB12: trouble sleeping 7.6%, irritability 3.4%, hyperactivity 11.9%; abstract: not significantly different from placebo
    Certainty: Injected doses far above dietary or usual supplement intake, autism only; the same review reports a multivitamin trial with B12 that improved sleep (n=20), so direction is unsettled.
    Journal of Personalized Medicine, 2021 · checked 2026-10-07 · we read the fulltext