The Penn State study used physiologic sleep data to show a connection between obesity and depression or sleepiness. Study participants filled out a comprehensive sleep history and physical examination and were evaluated in a sleep laboratory. “Obesity and weight gain predicted who was going to have daytime sleepiness,” said Julio Fernandez-Mendoza, assistant professor of psychiatry at the Sleep Research and Treatment Center. “Weight loss predicted who was going to stop experiencing daytime sleepiness, reinforcing the causal relationship.”
Body mass index and sleepiness association was independent of sleep duration, so obese people might be sleepy during the day regardless of how much sleep they get. Obesity is also associated with sleep apnea. The chief reason heavy people are more tired is that fat cells create immune compounds called cytokines that make one sleepy.
According to the study, depressed people have daytime drowsiness because they have trouble falling asleep and often wake up during the night. “The mechanism that we believe is playing a role here is hyperarousal, which is simply going to bed and being to alert; in other words, people with depression feel fatigued but do not necessarily fall asleep during the day, Fernandez-Mendoza said.
The study showed that a one-size-fits-all method for treating daytime drowsiness will not be effective. Daytime sleepiness doesn’t always mean a person doesn’t get enough sleep, Fernandez-Mendoza said. “The main causes of a sleepy society are an obese society, a depressed society and, to some extent, people who have a physiological disorder. By looking at our patients more closely, we can start personalizing sleep medicine.”