Vitamin D is one of the most widely used supplements, and most people take it for their bones. A new study suggests it may also be tied to something else: how well people with early memory problems perform on a cognitive test. The finding is small, and it comes with a catch you should know about before reaching for a bottle. Here is what the researchers found, what it does and doesn’t prove, and what I would tell my own patients.
Can Poor Sleep Make Mild Cognitive Impairment Worse?
Mild cognitive impairment (MCI) sits between normal age-related memory changes and early dementia. People with MCI notice problems with memory or thinking, and cognitive tests pick up those changes, but they can still handle daily life. About 20% of older adults have it.
Not everyone with MCI develops Alzheimer’s disease, but the risk is higher than in the general population. That makes this stage a point where doctors can step in and try to protect brain function.
Sleep complicates the picture. Among people with MCI, about 36% report sleep problems, and that number rises to 46% when sleep is measured objectively. Poor sleep is linked to worse thinking skills in the years that follow, and the connection runs in both directions. MCI can disrupt sleep, and disrupted sleep can make cognitive problems worse. Researchers have proposed two explanations: inflammation in the brain and a buildup of amyloid beta, the protein that forms the plaques seen in Alzheimer’s disease.
Vitamin D has been studied on both fronts. Some research ties it to better cognition in older adults with MCI and to better sleep quality, but the results across studies are mixed. No one had looked at vitamin D in people who have both MCI and sleep problems. A team led by sleep scientist Victoria Pak at Emory University set out to fill that gap.
The Study at a Glance
The study included 54 adults aged 50 or older. All had diagnosed or suspected MCI, and all showed signs of disturbed sleep. The average age was 67, and 46% of participants were men. About 23% had less than a college-level education.
Researchers measured cognition with the Montreal Cognitive Assessment (MoCA), a short screening test for MCI scored out of 30. They identified sleep problems with two questionnaires. The Pittsburgh Sleep Quality Index measures overall sleep quality, and the Epworth Sleepiness Scale measures daytime sleepiness. Participants qualified with a score of 5 or higher on the first or 10 or higher on the second. Nearly all (98%) were poor sleepers by the first measure, 43% were sleepy by the second, and 41% met both criteria.
Participants reported their vitamin D supplement use on a questionnaire, including the daily dose and whether they took D2 or D3. Trained research staff also recorded each person’s medical history, medications, and supplement use.
The researchers then compared MoCA scores across dose groups, using people who took no daily vitamin D as the reference. They adjusted the results for age, sex, body mass index (BMI), education, and vitamin D type, since each of these can affect cognitive scores on its own.
This was a cross-sectional study, which means it captured each person’s vitamin D intake and cognitive score at one point in time. Nobody was assigned to take vitamin D, and nobody was followed over time. Participants were already taking their own doses, so the study can show whether the two things are associated. It cannot show that one caused the other. The authors describe their work as exploratory.
Higher Vitamin D, Better Cognitive Scores
People who took 5,000 IU or more of vitamin D daily scored 3.78 points higher on the MoCA than people who took no daily vitamin D. That is about 13% of the test’s 30-point scale, and it is where the headline number comes from. For context, a score of 26 or above is generally considered normal, so a difference of nearly 4 points is meaningful on this test.
The result was close to the line, though. The authors call the association “marginally significant,” which means it barely passed the threshold researchers use to rule out chance.
Lower doses did not show the same effect. People taking smaller daily amounts scored slightly higher than those taking none, but the difference was too small to rule out chance. The type of vitamin D made no difference either. D2 and D3 users had similar results at the higher dose.
Vitamin D was also not the only factor linked to better scores. Participants with a college degree scored 3.14 points higher, and participants who were overweight scored 3.59 points higher, both statistically significant. Education is known to affect performance on cognitive tests, and the authors list body weight among the factors that may shape these results. The vitamin D finding sits alongside other influences of similar size.
A Dose of Caution
The dose linked to better scores sits above the established safety limit. The tolerable upper intake level for vitamin D is 4,000 IU per day for everyone older than 8. The recommended daily amount is much lower: at least 600 IU for adults aged 50 to 70, and at least 800 IU for adults over 70.
The upper limit exists to prevent hypercalcemia, which is too much calcium in the blood. Vitamin D increases how much calcium your body absorbs, and excess calcium can cause nausea, vomiting, weakness, and kidney problems.
This is why the study doesn’t support raising your dose. The researchers did not propose a new recommendation, and readers shouldn’t draw one from it. The “5,000+ IU” group also includes anyone taking that amount or more, so the study can’t tell us how high those doses went. Participants reported their own intake, which adds more uncertainty.
Some other research suggests that doses between 4,000 and 6,000 IU may offer greater protection against certain health problems. That work is still emerging, and it has not changed official guidance.
If you take vitamin D, stay at or below 4,000 IU per day unless your doctor directs otherwise. If you’re considering a higher dose, ask your doctor to check your vitamin D blood level first. The right dose depends on where your level starts, and taking more than you need carries risk without a proven benefit.
My Personal RX on Protecting Your Brain and Sleep After 50
Memory lapses and poor sleep often show up together after 50. You can’t control every risk factor, but several daily habits are within your reach. These are the steps I recommend to my patients to support brain health and better sleep.
- Check Your Vitamin D Level:Β A simple blood test shows whether you’re low. Your result, not a guess, should set your dose.
- Stay Within the Safe Limit:Β Unless your doctor directs otherwise, keep supplemental vitamin D at or below 4,000 IU daily, counting every supplement you take.
- Get Outside in the Morning:Β Daylight helps your skin make vitamin D and keeps your sleep-wake cycle on track.
- Keep a Steady Sleep Schedule:Β Go to bed and wake up at the same times every day, weekends included.
- Build a Wind-Down Routine:Β Dim the lights and put screens away an hour before bed. If you still struggle to fall asleep,Β Sleep MaxΒ combines magnesium, GABA, 5-HTP, and taurine to calm your mind and support restorative REM sleep.
- Fill Nutrient Gaps After 40:Β TheΒ Core Four BundleΒ combines omega-3 fish oil, Ultra Cal-Mag, vitamin D3 + K2, and MindBiotic to cover common shortfalls.
- Move Every Day:Β Regular walking supports circulation and improves sleep quality.
- Challenge Your Brain:Β Read, learn a language, or play strategy games. Education was linked to higher scores in this study, so keep learning.
- Cut Back on Evening Alcohol and Caffeine:Β Both disrupt sleep, even when they don’t keep you awake.
- Learn the Gut-Brain Connection:Β My bookΒ Heal Your Gut, Save Your BrainΒ explains how gut health affects mood and thinking, and how to improve both.
Source:
- Dockrill, P. (2026, August 25).Β Common Daily Vitamin Linked to 13% Cognitive Boost in Early Dementia, Study Shows. ScienceAlert. https://www.sciencealert.com/common-daily-vitamin-linked-to-13-cognitive-boost-in-early-dementia-study-shows
- Zhou, S., Sudeshna, P., Trotti, L. M., Bliwise, D., & Pak, V. (2026). Vitamin D supplement intake is associated with better cognition in persons with sleep disturbance and mild cognitive impairment.Β Sleep Medicine,Β 146, 108960. https://doi.org/10.1016/j.sleep.2026.108960
