This is the first trial to investigate the effects of vitamin D supplementation on sleep quality in individuals without other specific medical conditions. The increase in vitamin D levels in the blood of participants in the vitamin D group confirmed that the participants actually took the pills and that they were effectively absorbed. This is a promising exploratory trial that provides suggestive evidence that supplemental vitamin D may help improve sleep in younger people who are vitamin D insufficient without other identifiable sleep problems. Although the study has limitations, it may be enough to warrant future, larger, more carefully focused investigations.
One such limitation is the clinical relevance of its findings, since it is difficult to determine what the decrease of 2.7 in total sleep quality score found in this study would mean at the practical level. The PSQI questionnaire uses a cut-off of five or more to define poor sleep and, by this measure, the average post-intervention score of 6.75 would mean those in the vitamin D group would still be considered to have poor sleep at the end of the study, on average. However, an improvement of 2.7 in the total sleep quality score is more than half the score needed to be classified as a poor sleeper, so it’s quite possible that this could make a practical impact on sleep quality for some people. However, the results presented in the study under review make this hard to know for certain.
The use of the PSQI as the main metric in this study leads to some other issues concerning study interpretation. Participation was based on an overall PSQI sleep score of five or above. While this is a reliable method of classifying people as poor sleepers, the information it yields is not very detailed. This means that it’s difficult to determine why they were sleeping poorly to begin with, or even whether the types of sleep problem differed between the vitamin D and placebo groups. This means that we cannot say from this study whether the potential benefits of vitamin D to sleep apply to any specific sleep problem, such as restless leg syndrome, or sleep apnea.
A limitation of using the PSQI is that it relies solely on self-reported data to evaluate the participants’ sleep quality. However, a recent meta-analysis concluded that the PSQI questionnaire is the most commonly used generic sleep measure in clinical and research settings and its results correlate well with other measures of sleep quality, such as a clinical diagnosis of insomnia. While the study sufferers from problems common to self-reported data, it appears to be somewhat reliable. More objective evidence could be obtained in the future by recording participant sleep in a sleep lab.
There’s also a question about the generalizability of this study’s results. A vitamin D blood level of between 20-29 ng/ml can be considered as insufficient, and levels less than 20 ng/ml as deficient, although there is not universal agreement on normal vitamin D levels. Participants had vitamin D levels below 30 ng/ml at the start of the study, suggesting that their vitamin D levels were low, but that they were not deficient. This means any benefits of supplemental vitamin D on sleep may not occur in people with already high levels and raises the possibility that the effects may have been larger in people with lower vitamin D status, although this remains speculative.
High dose vitamin D supplements were used in this study, containing 50,000 units of vitamin D taken once every two weeks. While this is beneficial in helping participants maintain compliance with the study, as they don’t have to remember to take a supplement everyday, it may not be as effective as daily supplements. One study reported that a daily dose of vitamin D was more effective than weekly or monthly doses at correcting vitamin D deficiency. Thus, it’s possible that daily supplementation may more effectively increase vitamin D levels compared to less frequent dosing. This, in turn, may also lead to different effects on sleep quality, and would also more closely mimic how a lot of people actually take vitamin D. Future studies using daily dosing are warranted.
This study was relatively short for studies involving vitamin D supplementation, as the level of this vitamin accumulates gradually in the body. Previous studies have found that vitamin D levels in the blood increase and then plateau after supplementation in people deficient in vitamin D. This would indicate that the participants were only just reaching their new higher level of vitamin D by the final week of the study, potentially limiting any beneficial effects. A longer study with more participants would be useful in addressing this.
This study tells us that the vitamin D supplementation may improve sleep quality in people who have moderately low levels to begin with. However, this study cannot say how vitamin D might benefit sleep or whether these benefits may extend to people with specific sleep problems. While these benefits for sleep quality were statistically significant and suggest a meaningful benefit, determining the real world relevance of these improvements will require more research.