If you frequently doze off during the day, low vitamin D could be a culprit. A recent study found that people who are chronically tired tend to have lower levels of D than those who experience only infrequent or mild daytime fatigue - and many Americans are low in the essential vitamin. Dr. David McCarty, lead study author and professor of sleep medicine at Louisiana State University Health, says running short on vitamin D may make it more likely you'll develop a sleep disorder, like insomnia or sleep apnea. But, he says, further research is needed to figure out whether taking a simple supplement could help. In the meantime, if you do get excessively tired during the day, you may want to try supplementing to see if it helps or load up on the few foods high in the vitamin, such as salmon and eggs.