Sleep deprivation makes the day drag and appears to put a drag on metabolism too, causing the body to use less energy, according to a European study.

The results, reported in the American Journal of Clinical Nutrition, add to evidence that sleep loss can promote weight gain - not just by boosting hunger but also by slowing the rate at which calories are burned.

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

'Our findings show that one night of sleep deprivation acutely reduces energy expenditure in healthy men, which suggests sleep contributes to the acute regulation of daytime energy expenditure in humans,' he wrote.

Previous studies have linked sleep deprivation with weight gain and also 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 have these effects, 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.

They found that even a single night of missed sleep slowed metabolism the next morning, reducing energy expenditure for tasks like breathing and digestion by 5 per cent to 20 per cent, compared with the morning after a good night's sleep.

The young men also had higher morning levels of blood sugar, appetite-regulating hormones such as ghrelin, and stress hormones such as cortisol after sleep disruption. 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 sleep five hours or less are more prone to weight gain and weigh-related diseases such as type-2 diabetes. But those studies do not prove that sleep loss causes weight gain.

Experts said that factors such as lifestyle and diet might add to obesity risks and that it was not clear that sleep deprivation led to obesity.

Sanford Auerbach, head of the Sleep Disorders Center at Boston Medical Center, noted that sleep deprivation is a complex issue, with medication and other issues influencing sleep as well, and urged that the new findings be kept in context.

'They showed that we adapt to sleep deprivation and that some of these adaptations could theoretically contribute to obesity,' he said, adding that it's not clear how chronic sleep loss influences hormone levels.

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