A study of adults with mild cognitive impairment and poor sleep gives readers a specific result to discuss with their clinicians: those reporting at least 5,000 IU of vitamin D daily had higher Montreal Cognitive Assessment scores. The finding is preliminary because the study included 54 people, measured supplements at one point, and cannot show that vitamin D caused the difference.
What did the study find?
Researchers studied adults who had diagnosed or suspected mild cognitive impairment, or MCI, and disturbed sleep. They used the Montreal Cognitive Assessment, or MoCA, to assess cognition.
The study in Sleep Medicine found that participants reporting 5,000 IU or more of vitamin D each day had an adjusted mean MoCA score 3.78 points higher than participants who did not take a daily supplement. The result was described as marginally significant, with a p-value of 0.039. The original report described a 13% improvement after adjustment.
Lower daily doses were not significantly associated with MoCA scores. The analysis also found no significant difference between vitamin D2 and vitamin D3 users. A college degree was associated with an adjusted mean score that was 3.14 points higher, while being overweight was associated with a score 3.59 points higher. Treat these as reported associations rather than proof that education or weight protects cognition.
Who took part?
The sample contained 54 adults with a mean age of 67.2 years. Their mean body mass index was 29.5, and 46% were male. Disturbed sleep was defined by a Pittsburgh Sleep Quality Index score of at least 5, an Epworth Sleepiness Scale score of at least 10, or the study’s stated criteria for disturbed sleep.
Participants reported their supplement use through a questionnaire. The researchers used multiple linear regression to account for age, sex, body mass index, vitamin D form and education. This helps separate the reported supplement association from those measured characteristics, but it cannot remove every possible source of confounding.
Why does this finding need caution?
The study was cross-sectional. Researchers assessed cognition and asked about supplement use during the same analysis period. They did not follow participants to see whether vitamin D use preceded changes in cognition, and they did not randomly assign participants to a supplement or placebo.
The study also focused on supplements. It did not analyze vitamin D from food or sunlight. The sample was small and limited to adults with MCI and disturbed sleep, so it cannot be applied directly to people without those conditions.
The reported difference also sat near the conventional statistical threshold. The study authors suggested that taking higher doses of daily vitamin D supplementation may be associated with better overall cognitive performance in this population. The paper does not establish that 5,000 IU per day is a recommended dose or that vitamin D prevents dementia.
How does this fit with other research?
Other studies have not reached the same conclusion. A 2025 study published in Nutrients enrolled 718 adults aged 55 to 80, including 470 with normal cognitive function and 248 with MCI. It found no significant relationship between vitamin D status and cognitive function. Vitamin D inadequacy, defined as less than 30 ng/mL, was present in about 40% of the participants in both groups.
A separate randomized trial reported in a 2024 preprint included 620 adults aged 50 or older with mild to moderate vitamin D deficiency and early cognitive impairment. Over 24 months, vitamin D supplementation did not significantly improve executive function, other cognitive measures, function or wellbeing compared with placebo. The researchers said the trial found no measurable improvement in cognitive outcomes, and the result remains a preprint.
The SYNERGIC randomized clinical trial in JAMA Network Open enrolled 175 adults aged 65 to 84 with MCI. Participants received a 20-week intervention involving aerobic-resistance exercise, computerized cognitive training, vitamin D, placebo or a combination. Vitamin D was given at 10,000 IU three times weekly. At six months, aerobic-resistance exercise improved ADAS-Cog-13 scores compared with control, but vitamin D alone did not significantly improve cognition. Exercise plus cognitive training improved the score compared with exercise alone.
A 2020 review in Journal of Aging Research described vitamin D as a neurosteroid with anti-inflammatory, antioxidant and neuroprotective properties. It also noted that observational studies associate low vitamin D with cognitive impairment and dementia, while intervention trials have produced mixed results. The review said that existing randomized trials have had small samples and no consensus on dose or age at which supplementation should begin.
A 2022 systematic review in Nutrients concluded that vitamin D trial results were not conclusive for assessing possible cognitive benefits. The review also found more encouraging results for some B vitamins, but the findings did not establish a general benefit from vitamin D supplementation.
What to do now
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If you have MCI or sleep problems, discuss the finding with a clinician rather than starting a high dose on your own.
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Ask whether your vitamin D level has been checked and whether your current supplement dose is appropriate for you.
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Tell the clinician which vitamin D form you use, how much you take daily, and when you started it.
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Discuss monitoring cognition and sleep with a clinician.
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Keep following any exercise or cognitive-care plan already recommended by your clinician.
What this means for you
The most useful takeaway is a narrower one than the headline can suggest. A reported daily dose of 5,000 IU was associated with higher cognitive screening scores in 54 adults who already had MCI and disturbed sleep. Randomized evidence cited above did not show a significant cognitive benefit from vitamin D, and one larger observational study found no relationship between vitamin D status and cognition.
A supplement decision should therefore be based on your medical history, laboratory results and clinician’s advice. The finding may help guide a conversation, but it does not establish a treatment for MCI or a dose that consumers should adopt.
FAQ
Does vitamin D improve cognition in adults with MCI and poor sleep?
The 2026 study found an association between taking at least 5,000 IU daily and higher MoCA scores. Because it was a small cross-sectional study, it cannot show that vitamin D caused the higher scores or that supplementation improves cognition.
What dose of vitamin D was linked with higher cognitive scores?
Participants reporting at least 5,000 IU daily had an adjusted mean MoCA score 3.78 points higher than participants who did not take a daily supplement. Ask a clinician whether any dose is appropriate for you.
Did randomized trials find that vitamin D improves cognition?
The VitaMIND randomized trial found no measurable improvement in cognitive outcomes compared with placebo. In the SYNERGIC trial, vitamin D did not significantly improve cognition, while aerobic-resistance exercise did.
Who was included in the 2026 study?
The study included 54 adults with diagnosed or suspected MCI and disturbed sleep. Their mean age was 67.2 years, mean BMI was 29.5, and 46% were male.
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