Sleep deprivation makes the day drag and appears to put a drag on metabolism, causing the body to use less energy, European researchers found in a small study.

The results, which appear in the American Journal of Clinical Nutrition, add to evidence that a lack of shut-eye can promote weight gain -- not just by boosting hunger, but by slowing the rate at which calories get burned.

The study suggests that getting plenty of sleep might prevent weight gain, said Dr. Christian Benedict of the Uppsala University in Sweden, who led the new work.

Approximately 50 to 70 million Americans - including a significant number of shift workers -- suffer from chronic sleep loss and sleep disorders, according to the National Institutes of Health.

Previous studies have linked sleep deprivation with weight gain, and shown how disrupted sleep also disrupts levels of stress- and hunger-related hormones during waking hours.

To help identify the exact mechanisms by which a lack of sleep might pack on the pounds, Benedict and his colleagues put 14 male university students through a series of sleep "conditions" - curtailed sleep, no sleep, and normal sleep - over several days, then measured changes in how much they ate, their blood sugar, hormone levels and indicators of their metabolic rate like oxygen use.

The team found that even a single night of missed sleep slowed the volunteers' metabolisms the next morning, reducing their bodies' energy expenditure for tasks like breathing and digestion by 5 to 20 percent, compared with the morning after a good night's sleep.

The young men also had higher morning levels of blood sugar, appetite-regulating hormones like ghrelin, and stress hormones like cortisol after sleep deprivation. Still, the sleep loss did not boost the amount of food the men consumed during the day.

A number of studies have observed that people who snooze five hours or less are more prone to piling on weight and developing weight-related diseases such as type 2 diabetes.

But those studies do not prove that sleep loss causes weight gain.

"It is not clear that sleep deprivation leads to obesity," Dr. Carol Everson, who was not involved in the German study, told Reuters Health by email.

Factors such as lifestyle and diet may also add to obesity risk, said Everson, a sleep expert at the Medical College of Wisconsin.

It's tempting to link sleep health and obesity but sleep deprivation is complicated, added Dr. Sanford Auerbach, who heads the Sleep Disorders Center at Boston Medical Center. For instance, medications and other conditions may influence sleep, he said.

The new findings, Auerbach noted in an email, should be kept in context. "They showed that we adapt to sleep deprivation and that some of these adaptations could theoretically contribute to obesity." On the other hand, it's not clear how chronic sleep loss influences hormone levels, he said.

The National Sleep Foundation recommends that adults get about seven to nine hours of sleep each night.

For those who don't get their full 40 winks and worry about weight gain, Benedict recommends they commit to a daily mealtime to help keep the extra weight at bay.

SOURCE: bit.ly/gyfkpT The American Journal of Clinical Nutrition, online April 6, 2011.