High-Dose Vitamin D Linked to Better Cognitive Test Scores in People With Sleep Problems

A small study by scientists at Emory University suggests that people with mild cognitive impairment and sleep disturbances—two potential signs of early dementia—may perform better on standard cognitive tests if they take more than 5,000 international units (IU) of vitamin D daily.

Vitamin D, sleep and cognitive health

Vitamin D is an essential nutrient required for muscle and nerve function. It also influences sleep quality and sleep-wake cycles.

Sleep disturbances are common among people with cognitive impairment. About 50% of patients with moderate to severe Alzheimer’s disease report sleep problems, suggesting a bidirectional relationship between sleep deprivation and cognitive decline.

“Sleep disorders affect nearly half of patients with mild cognitive impairment, who are at high risk of progressing to Alzheimer’s disease,” said lead author Dr. Victoria Pack, a researcher at Emory University, and her colleagues.

“The role of vitamin D in cognitive outcomes, particularly in people with sleep disorders and mild cognitive impairment, remains unclear,” the researchers said.

What the vitamin D study found

The study examined 54 people diagnosed with, or suspected of having, mild cognitive impairment—a stage between normal aging and dementia—who also reported sleep problems.

Participants completed the Montreal Cognitive Assessment, a widely used screening test for cognitive function. They also reported their vitamin D supplement use in a questionnaire.

Researchers found that participants who took more than 5,000 IU of vitamin D each day had higher cognitive test scores than those who did not take vitamin D.

The researchers also found no difference in cognitive performance based on the type of vitamin D supplement. Vitamin D2 usually comes from plants and fungi, while vitamin D3 is produced after exposure to sunlight and is also found in animal foods.

Early intervention and Alzheimer’s disease risk

“For older adults who experience both sleep disorders and mild cognitive impairment, when cognitive changes are emerging, this can be an important opportunity for intervention, but there may still be opportunities to support brain health,” Dr. Park said.

“Identifying available and modifiable factors, such as taking vitamin D supplements, in the early stages of cognitive decline may become increasingly important, especially as the incidence of Alzheimer’s disease continues to rise.”

Because this was a small study based on reported supplement use, the findings show an association between higher vitamin D intake and cognitive test performance rather than proving that vitamin D improves cognition.

The study is listed for publication in the October 2026 issue of the journal Sleep.

Read the study

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Zhou Si Rui et al. 2026. Vitamin D supplement intake is associated with improved cognitive performance in people with sleep disorders and mild cognitive impairment. Sleep 146: 108960. doi: 10.1016/j.sleep.2026.108960