Vitamin D3 and Sleep

In recent years a number of publications have appeared about the link between vitamin D and sleep: people with a low level of the vitamin more often complain of insomnia and daytime sleepiness. From this came advice to take D3 «for sleep» — or, on the contrary, never to take it in the evening. The editorial team looked into what of this is confirmed and what remains a hypothesis.

Where the topic of vitamin D and sleep came from

Interest in the link between vitamin D and sleep arose largely after the publication of the hypothesis by Gominak and Stumpf (2012). The authors, drawing on clinical observations, suggested that the prevalence of sleep disorders might be related to vitamin D deficiency. It is important to emphasize: this was precisely a hypothesis, published in a journal that specializes in theoretical assumptions, and not the result of a controlled study.

The topic was subsequently developed by epidemiologists. A systematic review and meta-analysis by Gao and colleagues (2018) pooled observational studies and concluded that vitamin D deficiency is associated with an increased risk of sleep disorders, in particular with poorer sleep quality, shorter duration, and daytime sleepiness.

However, observational studies have a known limitation: they show an association but not a cause. Low vitamin D often accompanies obesity, a sedentary lifestyle, little time outdoors, chronic diseases, and depression. Each of these factors in itself worsens sleep, and separating their influence statistically is very difficult.

There is also a reverse link: people who sleep poorly often have less daytime activity and spend less time in daylight. That is, poor sleep can lead to low vitamin D, and not only the other way around.

Biological mechanisms: what is known

There is a biological basis for the hypothesis. A study by Eyles and colleagues (2005) found the vitamin D receptor and the enzyme that forms its active form in many areas of the human brain, in particular in the hypothalamus, which is involved in regulating the sleep-wake cycle. This means that vitamin D can theoretically affect the neurons that control sleep.

Other hypothetical mechanisms include vitamin D's participation in the synthesis of serotonin and melatonin, its effect on inflammatory processes and on musculoskeletal pain. Chronic pain and inflammation are known causes of poor sleep, so correcting a severe deficiency accompanied by pain in the bones and muscles may logically improve sleep as well.

There is also an opposite hypothesis, popular on the internet: that vitamin D supposedly suppresses melatonin production, and therefore it cannot be taken in the evening. This idea stems from the fact that both substances are linked to light: vitamin D is formed during the day under the sun, melatonin — in darkness. However, there is no direct convincing evidence that oral vitamin D in the evening lowers melatonin in humans.

So, the mechanisms are plausible, but they have mostly been studied at the level of receptor localization and theoretical models. From this to a proven clinical effect is still a long way.

What human studies show

There are comparatively few randomized controlled trials in which sleep was the main outcome, and they are small. Some of them reported an improvement in subjective sleep quality in people with vitamin D deficiency who received a supplement, especially among patients with certain sleep disorders or chronic diseases. Other studies found no advantage over placebo.

The overall assessment of systematic reviews of RCTs is that the evidence is limited and heterogeneous. Doses, duration, populations, and methods of assessing sleep differ: from questionnaires to polysomnography. Small sample sizes and subjective scales make the results sensitive to the placebo effect.

For athletes there is almost no specific research on sleep. There are only general data that sleep is critically important for recovery, and vitamin D — for the health of bones and muscles. Combining these two facts into a guarantee of «better sleep from D3» is incorrect.

A realistic formulation is this: if a person has a pronounced vitamin D deficiency, correcting it may have a positive effect on general well-being, pain, and, possibly, sleep. But if the status is normal, a supplement is unlikely to improve sleep.

Morning or evening: does the timing matter

One of the most frequent questions is when to take vitamin D so as not to worsen sleep. The common advice «only in the morning» is based on an analogy with natural synthesis under the sun and on the hypothesis about melatonin suppression. There is no reliable clinical evidence for this advice.

From a pharmacological point of view, vitamin D3 after intake is gradually converted in the liver into 25(OH)D, whose half-life is about two to three weeks. That is, the blood level changes slowly and practically does not depend on what hour the capsule was taken. Vitamin D has no acute «daily» effect like caffeine or melatonin.

Much more important is to take vitamin D regularly and together with food that contains fats, since this improves its absorption. For most people the most convenient time is during the most substantial meal.

• The time of day is of no fundamental importance for the 25(OH)D level.
• Taking it with food that contains fats improves absorption.
• If you subjectively notice worse sleep after an evening intake, simply move it to the morning.
• Regularity is more important than the exact hour.

Separately, it is worth mentioning combined «sleep» products in which vitamin D is combined with melatonin or plant extracts. Their effect is determined primarily by these components, not by vitamin D, and such complexes should be assessed taking each ingredient into account.

Practical conclusions for athletes

If an athlete sleeps poorly, the first step is to assess the main factors: the training schedule (intense workouts late in the evening can interfere with falling asleep), caffeine intake, in particular in pre-workout complexes, screen light before bed, the temperature in the bedroom, stress, and the overall duration of sleep.

The consensus of the American Academy of Sleep Medicine recommends that adults sleep at least seven hours a day, and athletes during periods of high loads often need more. Behavioral changes in this area give a much greater effect than any supplement.

It is appropriate to check vitamin D if, in addition to sleep problems, there are other risk factors for deficiency: little sun, the winter season, indoor training, pain in the bones or muscles, frequent stress fractures. In that case, correcting the deficiency is useful regardless of its effect on sleep.

But if the sleep disturbances are prolonged — chronic insomnia, snoring with breathing pauses, strong daytime sleepiness — a doctor's consultation is needed. Conditions such as obstructive sleep apnea syndrome require special diagnosis and cannot be «cured» with vitamins.

Editorial conclusions

Observational studies consistently link low vitamin D with worse sleep, and vitamin receptors are present in brain areas that regulate sleep. However, a cause-and-effect relationship has not been proven, and randomized studies are small and contradictory.

Correcting a deficiency may make sense for general health and, possibly, for sleep, but for people with normal status a supplement is unlikely to help them sleep better. The timing is of almost no importance for the vitamin level in the blood — regularity and taking it with food are more important.

The foundation of healthy sleep remains a routine, control of caffeine and light, and, for prolonged problems, a medical examination.

We also recommend reading our materials «Magnesium and sleep», «Vitamin D3 after 40», and «Vitamin D3 during a bulk: does it make sense».

References

1. Gao Q, Kou T, Zhuang B, et al. The association between vitamin D deficiency and sleep disorders: a systematic review and meta-analysis. Nutrients. 2018;10(10):1395.
2. Gominak SC, Stumpf WE. The world epidemic of sleep disorders is linked to vitamin D deficiency. Med Hypotheses. 2012;79(2):132–135.
3. Eyles DW, Smith S, Kinobe R, Hewison M, McGrath JJ. Distribution of the vitamin D receptor and 1α-hydroxylase in human brain. J Chem Neuroanat. 2005;29(1):21–30.
4. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266–281.
5. Watson NF, Badr MS, Belenky G, et al. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep. 2015;38(6):843–844.
6. Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Med. 2018;48(Suppl 1):3–16.