New research suggests that obesity and depression -- not only lack of sleep -- are underlying causes for regular drowsiness.

Researchers at Penn State College of Medicine believe these findings could lead to more personalized sleep medicine for those with excessive daytime sleepiness.

As much as 30 percent of the general population experiences excessive daytime sleepiness -- daytime drowsiness or sleepiness occurring throughout the day that can include irresistible sleep attacks. Feeling overly tired during the day can reduce job productivity and increase errors and absenteeism and may lead to more serious issues like automobile accidents.

"Obesity and weight gain predicted who was going to have daytime sleepiness," Julio Fernandez-Mendoza, researcher and assistant professor of psychiatry at the Sleep Research and Treatment Center at Penn State College of Medicine, said in a statement. "Moreover, weight loss predicted who was going to stop experiencing daytime sleepiness, reinforcing the causal relationship."

For the study, researchers measured self-reporting of excessive daytime sleepiness at baseline and again an average of 7.5 years later in more than 1,000 men and women. Study participants completed a comprehensive sleep history and physical examination and were evaluated for one night in a sleep laboratory. The researchers also recorded sleep, physical and mental health problems and substance use and determined whether participants were being treated for physical and mental health conditions.

Researchers found that body weight predicted excessive daytime sleepiness better than sleep apnea.

"This data is also consistent with studies showing that CPAP (continuous positive airway pressure) machines greatly reduce the number of apneas, or pauses in breathing, that a person with sleep apnea experiences during the night, but don't effectively reduce daytime sleepiness, probably because CPAP does not help reduce weight," Fernandez-Mendoza explained.

The findings are detailed in the journal Sleep.