Doctors commonly view excessive daytime sleepiness as a cardinal sign of disturbed or inadequate sleep. But a new study suggests it could also signal depression or even diabetes, regardless of whether an individual doesn’t sleep well. 

Among a random sample of 16,500 men and women ranging in age from 20 to 100 years old from central Pennsylvania, 8.7 percent had excessive daytime sleepiness. 

Researchers, who considered a wide range of possible reasons for why these individuals were excessively sleepy during the daytime, found that excessive daytime sleepiness was more strongly associated with depression and obesity or metabolic factors than with sleep-disordered breathing or sleep disruption. 

Depression was by far the most significant risk factor for excessive daytime sleepiness, they report in the Journal of Clinical Endocrinology and Metabolism. 

The likelihood of being excessively sleepy during the daytime was more than three times higher in those who reported they were being treated for depression. 

The investigators also observed strong ties between excessive daytime sleepiness and diabetes. Individuals reporting treatment for diabetes were close to two times more likely to report excessive daytime sleepiness than those who were not being treated for diabetes. 

Being overweight also increased the likelihood of excessive daytime sleepiness. 

Excessive daytime sleepiness was more common in people younger than age 30, a finding that hints at the presence of unmet sleep needs and depression, and in the over-75 crowd, suggesting increasing medical illness and health problems, they explain. 

Smoking also emerged as a risk factor for excessive daytime sleepiness, a link that hasn’t been shown before. It could be that smokers use the stimulant effect of nicotine to self-treat their daytime drowsiness, the authors suggest. 

Sleep apnea -- brief episodes when breathing stops during sleep -- was not a significant player in excessive daytime sleepiness. This is consistent with prior studies that have reported only weak associations between sleep apnea and excessive daytime sleepiness. 

The authors conclude that adults plagued by excessive daytime sleepiness should be thoroughly evaluated for depression and diabetes, regardless of whether or not sleep-disordered breathing is present. 

SOURCE: The Journal of Clinical Endocrinology and Metabolism, August 2005.