People with more vitamin D in their blood slept a little longer, wrist sensors showed

More than half of older adults in the United States report trouble sleeping, and poor sleep is linked to many health problems. Low levels of vitamin D and vitamin B12 are also common at older ages. A new study asked whether the two go together, and this time measured both things directly: the vitamins in the blood, and sleep with a sensor worn on the wrist.

The answer was a modest link for vitamin D: people with the most of it in their blood slept about 19 minutes longer per night than those with the least, and their sleep was a bit less broken up. For vitamin B12 there was no clear overall link. The study is observational: it compares people, it did not give anyone vitamins, so it cannot say that vitamin D improves sleep.

Why measure instead of ask

Earlier studies mostly asked people how well they slept, and often used "takes a supplement" as a stand-in for vitamin levels. Both are rough. People misjudge their own sleep, and taking a supplement can mean someone was low in a vitamin, or that they now have plenty. The team behind this study therefore used blood tests and actigraphy: a watch-like device that records movement and light, from which validated software estimates when a person is asleep or awake.

What they did

The participants came from the Baltimore Longitudinal Study of Aging, a long-running US study of generally healthy volunteers. The analysis included 460 people aged 40 and older (average age 71, about half women, 22% Black) who had blood tests after fasting and wore the wrist sensor for up to seven days and nights between 2012 and 2019. People with dementia or milder memory problems were left out, because those conditions affect both sleep and diet. About a quarter of the participants were measured on two or more visits.

The team looked at five sleep measures: total sleep time, time awake after first falling asleep, the share of time in bed spent asleep, how long it took to fall asleep, and the average length of each waking spell. The results were adjusted for age, sex, race, education, number of illnesses, depressive symptoms and body weight.

On average, participants slept about 6.7 hours a night and lay awake for about 48 minutes after first falling asleep. Nearly half reported taking a vitamin D supplement. About 30% had vitamin D levels below 30 ng/ml (counted as insufficient or deficient), but only 14 people (3%) were low in B12.

What they found

More vitamin D, longer sleep. People in the top third of vitamin D levels slept about 19 minutes longer per night than those in the bottom third, spent a slightly higher share of their time in bed asleep (by about 1.4 percentage points) and had shorter waking spells.
Deficiency and short sleep. Compared with people in the normal range (above 30 ng/ml), those who were deficient (below 20 ng/ml) slept about 23 minutes less per night.
Supplements didn't change the picture. The results stayed about the same when the team also took into account whether people used vitamin D supplements.
Possibly stronger in Black participants. In an exploratory analysis, higher vitamin D went with a higher share of time asleep among Black participants, while the link was weaker and less consistent among the others.
B12: no clear link. Vitamin B12 levels were not linked to any sleep measure overall. There were hints that the link might go in opposite directions in women and men, but the separate results for each sex were not statistically significant.

What to keep in mind

Links, not causes. The authors point out that blood vitamin levels may simply be markers of other things, such as how much time people spend outdoors, how active they are or their general health, which could also affect sleep. They note that low vitamin D may be a sign of declining health for which supplements bring little, if any, benefit.
Mostly a snapshot. Most people (74%) had only one visit with both measurements, so the team could not follow changes over time or rule out that sleep affects vitamin levels rather than the other way round.
A healthy group. The volunteers were relatively healthy and free of memory problems, so the results may not apply to less healthy people.
Too few with low B12. So few people were low in B12 that the study may have been too small to detect an effect, and the differences between women and men are tentative.
Small differences. About 20 minutes of sleep is a real but modest gap, and the sensor cannot tell sleep stages apart.
No advice. The study did not test supplements, and it gives no basis for taking any.

Why it matters

Sleep tends to get shorter and more broken with age, and poor sleep is linked to many problems in later life, so researchers are looking for factors that could be changed. This study confirms an earlier finding in older men, now in a group of both women and men, with objective measurements of both sleep and vitamins. The authors' own conclusion is careful: future studies are needed to find out whether keeping vitamin D at healthy levels actually improves sleep, which only a randomized trial could show.