New research published in Sleep found that low vitamin D levels were strongly associated with worse outcomes of sleep duration and efficiency in older adult men.
In the late 20th century, vitamin D researchers studying the brain discovered potential mechanisms for a role for vitamin D in sleep. More specifically, they found vitamin D receptors on neurons in the brain regions responsible for the initiation and maintenance of sleep.
Ostensibly, this suggests that vitamin D may bind to the receptors in these brain regions and play a role in sleep and sleep quality.
Research on vitamin D and sleep in human populations, however, is limited. These studies are mostly observational and based on subjective, or self-reported, measures of sleep outcomes, which is a fundamental limitation in this research.
In a first-of-its-kind study, researchers from various centers throughout the United States conducted a study that looked at the relationship between circulating vitamin D levels and objective, or direct, measurements of sleep outcomes.
The researchers looked at the data from the Osteoporotic Fractures in Men Study (MrOS) for their current study. MrOS was a prospective cohort study initiated in 2000 to examine how bone mass, bone geometry and lifestyle affect fracture risk and how fractures impact quality of life among men aged 65 years or older.
In 2003, a secondary study was conducted called the MrOS Sleep Study, which recruited 3,135 participants from the MrOS to investigate various sleep-related factors.
For the current study, the researchers looked at data from 2,966 men aged 68 years or older from the MrOS Sleep Study who had available vitamin D measurements.
Participants were assigned to four different groups according to their vitamin D levels: less than 20.3 ng/ml, 20.3 – 30.4 ng/ml, 30.5 – 40.5 ng/ml, and 40.6 ng/ml and higher.
Sixty percent of the participants had levels below 30 ng/ml, while only 10% were grouped as having levels above 40 ng/ml.
All participants in the MrOS Sleep Study were required to wear actigraphs on their wrist for 24 hours a day for an average of 5 days. An actigraph unit is a non-invasive way to measure rest and activity cycles.
As opposed to previous studies, which looked at self-reported measurements of sleep, these actigraphs allowed for direct measurements of various sleep outcomes.
The researchers were interested in three sleep outcomes:
- Mean nightly total sleep time measured as the time from sleep onset to awakening.
- Sleep efficiency measured as the percent of time sleeping while in bed.
- Minutes of wake after sleep (WASO) measured as the amount of time awake after sleep onset to the end of the last time asleep.
A total sleep time of 5 hours or greater was considered a good night’s sleep, 70% or more time asleep was considered an efficient night’s sleep, and 90 minutes or more WASO was considered an indicator of poor sleep.
Data was collected after an average of 5 days monitoring the participants sleep activity. This data was compared between the groups to see if vitamin D status was related to three variables of interest.
Here’s what the research team found from that comparison:
- In men, 12.3% had sleep duration less than 5 hours, 26.3% had inefficient night’s sleep (less than 70% of the night asleep), and 57.5% had a WASO of greater than 90 minutes.
- Men with a vitamin D level lower than 30.5 ng/ml had a 2.15 times increased risk for shorter sleep duration compared to those with a vitamin D level of 40.6 ng/ml or higher (P < 0.004).
- Men with a vitamin D level lower than 30.5 ng/ml had a 45-65% increased risk for lower sleep efficiency compared to those with a level of 40.6 ng/ml or higher (P = 0.004).
- Men with a vitamin D level lower than 20.3 ng/ml had a 45% increased risk for greater WASO compared to those with a level of 40.5 ng/ml or higher. This was no longer significant after adjusting for all confounding variables.
The researchers stated,
“In conclusion, we found that low levels of serums 25(OH)D in older men are associated with short sleep duration and poorer sleep efficiency. If vitamin D indeed play a causal role in poorer sleep, then low levels of serum 25(OH)D may put men at risk for poor sleep.”
The research team notes that while they accounted for a wide array of potential confounding variables, there may have been some not adjusted for which could impact the results.
One confounding variable in studies like these is sun exposure. This means that vitamin D may be a marker for sun exposure, and there could be other aspects of sunlight in addition to vitamin D contributing to these results.
Other limitations include the analysis of only older men which limits the ability to generalize to younger men or women, and the observational design of the study, which means we cannot say that low vitamin D levels cause poor sleep.
This research adds insight to the relationship between vitamin D and sleep with its analysis of objective rather than subjective measurements. Researchers should now conduct trials to determine how correcting for vitamin D deficiency may impact overall sleep quality and outcomes.