NEW YORK - A number of studies have linked chronic sleep deprivation to a heightened risk of obesity, diabetes and heart disease. Now, a small study suggests that low levels of physical activity during the day may partly account for the connection.

In a study of 15 healthy men, researchers found that a couple nights of grabbing only four hours of sleep caused the men to curtail their physical activity compared with days where they had gotten the standard eight hours the night before.

In contrast, there was no evidence that sleep loss altered blood levels of appetite-regulating hormones or caused the men to eat more the next day -- effects that have been seen in a number of previous studies.

The implication is that there may be a broader range of reasons for the link between sleep loss and weight and health, the researchers report in the American Journal of Clinical Nutrition.

Practically speaking, the findings offer adults another reason to get enough sleep.

For healthy adults, that means regularly getting seven to eight hours per night, lead researcher Dr. Sebastian M. Schmid, of the University of Luebeck in Germany, told Reuters Health in an email.

A number of large epidemiological studies have found associations between poor sleep and higher risks of obesity and other health problems. Since then, a few small studies done in the sleep lab have attempted to find the possible reasons for the connection.

In some, researchers have found evidence that sleep loss alters the regulation of the hunger hormones leptin and ghrelin, and may boost daytime appetite. Leptin, which helps regulate body weight, is secreted by fat cells; low blood levels of the hormone promote hunger, while increases tell the brain that the body is full and encourage calorie burning. Ghrelin is secreted by the stomach to boost appetite.

But another possibility is that sleep-deprived people are just too tired to be physically active during the day.

While that seems logical, apparently no human studies had examined the question before.

For the new study, Schmid and his colleagues had 15 healthy, normal-weight men go through two consecutive nights with four hours of sleep and two nights with eight hours of sleep.

After the first night, the men spent the day doing their normal activities, while wearing a wrist device that recorded their movements. After the second night, they came to the sleep lab, where they again wore the wrist devices and also had their levels of leptin and ghrelin measured and their calorie intake monitored.

The researchers found that, unexpectedly, the men showed no differences in their hormone levels, hunger or food intake after the four-hour night compared with the eight-hour night.

They were, however, less active after sleep-deprived nights -- devoting both fewer minutes to physical activity and a smaller proportion of that time to more-intense exercise.

When the men got eight hours of sleep, they spent an average of 25 percent of their active time performing higher-intensity exercise; that declined to about 22 percent with four hours of sleep.

Over time, such differences could affect a person's weight and general health, according to Schmid's team.

The findings do not mean that sleep loss has no effects on hunger hormones and appetite, as earlier studies have suggested that it does. However, Schmid said, the results do suggest that even modest sleep restriction -- so common in today's society -- reduces physical activity, while hormones and appetite are "less affected."

SOURCE: American Journal of Clinical Nutrition, December 2009.