Can't stop yawning at work? We know why!

Feeling overly tired during the day can reduce job productivity and increase errors and absenteeism.

If you feel tired and drowsy the whole day even after a good night's sleep, it could be due to obesity and depression, a new research has found.

Obesity and depression -- not only lack of sleep -- are underlying causes for regular drowsiness, the findings showed.

"Obesity and weight gain predicted who was going to have daytime sleepiness," said Julio Fernandez-Mendoza, assistant professor of psychiatry at the Penn State College of Medicine in the US.

"Moreover, weight loss predicted who was going to stop experiencing daytime sleepiness, reinforcing the causal relationship," Fernandez-Mendoza noted.

The association between body mass index and sleepiness was independent of sleep duration, meaning obese people may be tired during the day, no matter how much they sleep at night.

The primary underlying mechanism that makes obese people feel overly tired is likely low-grade chronic inflammation. Fat cells, particularly from abdominal fat, produce immune compounds called cytokines that promote sleepiness, among other effects.

The researchers measured self-reporting of excessive daytime sleepiness (EDS) at baseline and again an average of 7.5 years later in 1,395 men and women.

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.

These findings could lead to more personalized sleep medicine for those with EDS.

The researchers published their findings in the journal Sleep.