Getting enough sleep is essential for good health, but you can get too much of a good thing, especially when it's in the form of a long afternoon nap. Scientists at the University of Tokyo found that long naps increase the risk of metabolic diseases.

They found that napping for 40 minutes or longer was associated with a steep increase in the risk of developing metabolic syndrome, a combination of factors, including high blood pressure, high cholesterol, and high blood sugar, that increase the risk of heart disease and diabetes. Feeling tired during the day was also linked to an increased risk of metabolic syndrome.

"Taking naps is widely prevalent around the world," said Dr. Tomohide Yamada, diabetologist at the University of Tokyo and lead author of the study. "So, clarifying the relationship between naps and metabolic disease might offer a new strategy of treatment, especially as metabolic disease has been increasing steadily all over the world."

The new study evaluated data from 21 observational studies involving 307,237 Asian and Western subjects. It builds on earlier studies by Yamada and his colleagues that tied lengthy naps and daytime sleepiness to a greater prevalence of heart disease and Type 2 diabetes.

Subjects who napped for less than 40 minutes did not show any increased risk for metabolic syndrome, but the risk rose sharply if naps lasted longer than 40 minutes. Napping for 90 minutes increased risk of metabolic syndrome risk by as much as 50 percent.

Short naps, however, appeared to be beneficial: There was a slight decrease in the risk of metabolic syndrome among those who napped less than 30 minutes.

According to the Centers for Disease Control and Prevention, about one-third of American adults do not get enough sleep. Experts recommend seven to nine hours of sleep at night, but a Gallup poll found that Americans average 6.8 hours of sleep, a decrease of more than an hour from 1942. The National Sleep Foundation recommends naps of 20 to 30 minutes.

"Sleep is an important component of our healthy lifestyle, as well as diet and exercise," Yamada said. "Short naps might have a beneficial effect on our health, but we don't yet know the strength of that effect or the mechanism by which it works."