An extra hour in bed a night may help you to lose weight, research suggests.
The study, by the US National Institute of Mental Health, also suggests that too little sleep may increase the risk of obesity.
Research suggests a chemical released during sleep, leptin, controls body fat by signalling when we are full.
Experts, writing in the journal Sleep, also say that lack of sleep probably affects hormones which help burn off calories during the day.
Researchers monitored nearly 500 adults, aged 27 to 40, over 13 years.
During that time the average amount of sleep fell from 7.7 hours to 7.3 hours for women, and from 7.1 hours to 6.9 hours for men.
Over the same period the volunteers put on an average of 5lb in weight.
Those who put on the most weight slept for less than six hours a night.
Lead researcher Dr Gregor Hasler admitted that one factor might be that the longer people stay up the more likely they are to eat.
However, he added: "Another possible cause is that we could be playing havoc with the body's natural systems, which are linked to going to bed at night and getting a reasonable amount of sleep."
Dr Sanjay Patel, a sleep expert at Harvard Medical School, said it was known that various chemicals and hormones played a key role in controlling appetite and weight gain.
"They can be greatly altered if you alter the length of time you sleep by an hour or two," he said.
Dr Patel said previous research showed leptin levels were significantly reduced in volunteers who were deprived of sleep.
"Low levels would increase the urge to eat," he said.
Importance of sleep
Dr Neil Stanley, a sleep expert at the Human Psychopharmacology Research Unit at Surrey University, told BBC News Online: "We know that sleep is crucial for good physical and mental health."
He added that people who were sleep-deprived tended to crave high carbohydrate, high fat food.
Dr Stanley said there was little evidence in the old adage that everybody needed eight hours sleep a night.
"You need enough sleep to feel awake and alert during the day - that could be three hours, or 11 hours, everybody has their own individual sleep need," he said.
"But if you do sleep less than you need, then it is a health risk."
A survey published earlier this year found nearly three quarters of us are getting less sleep than we were five years ago.
The research, commissioned by Travel Inn, found 57% of us feel that lack of sleep affects our performance at least once a week.
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