Not getting enough sleep doesn’t just make you tired. It also makes you fat, according to scientists.
In a study, participants who slept for five hours each night gained two pounds in weight over a week because they snacked more.
They consumed more calories in the form of after-dinner snacks than in any other meal.
But when they shifted to adequate sleep patterns they reduced their consumption of fat and carbohydrate and shed the pounds.
The findings are alarming with millions of Britons – about one in three adults – regularly sleeping less than five hours each night.
During periods of insufficient rest the volunteers ate a small breakfast but consumed an increased number of calories in the form of snacks that were rich in carbohydrates, protein and fibre.
However, when they shifted to adequate sleep patterns the group reduced their consumption of fats and carbohydrates – and shed the pounds.
Professor Ken Wright, of Colorado Boulder University, said: ‘I don’t think extra sleep by itself is going to lead to weight loss.
‘Problems with weight gain and obesity are much more complex than that. But I think it could help.
‘If we can incorporate healthy sleep into weight-loss and weight-maintenance programmes our findings suggest it may assist people to obtain a healthier weight.’
However, he said that further research is needed to test the hypothesis.
A previous study discovered a lack of sleep can drastically slow the metabolism down which causes the body to use less energy for simple tasks like breathing and eating.
A lack of sleep is thought to encourage weight gain by boosting hunger, but also by slowing the rate at which calories are burned.
In the latest paper, which is published in Proceedings of the National Academy of Sciences, researchers showed that while staying awake longer requires more energy, the amount of food participants ate more than offset the extra calories burned.
Professor Wright said: ‘Just getting less sleep by itself is not going to lead to weight gain. But when people get insufficient sleep it leads them to eat more than they actually need.’
His researchers monitored 16 young, lean and healthy adults who lived for two weeks at the University of Colorado Hospital’s ‘sleep suite’.
During the first three days the volunteers were allowed to sleep for nine hours a night and eat meals controlled to provide only the calories needed to maintain their weight.
Then they were split into two groups for five days – one allowed only five hours sleep a night, and the other nine hours.
In both groups participants were offered larger meals and had access to snack options throughout the day ranging from fruit and yogurt to ice cream and crisps.
After the five-day period, the groups switched.
On average, the participants who slept for up to five hours a night burned five per cent more energy than those who slept up to nine hours a night, but they consumed six per cent more calories.
Those getting less sleep also tended to eat smaller breakfasts but binge on after-dinner snacks.
In fact, the total amount of calories consumed in evening snacks was larger than the calories that made up any individual meal.
The current findings add to the growing body of evidence showing that overeating at night may contribute to weight gain.
Professor Wright said: ‘When people are sleep-restricted our findings show they eat during their biological night-time when internal physiology is not designed to be taking in food.’
The researchers also found men and women responded differently to having access to unrestricted food.
Men gained some weight even with adequate sleep when they could eat as much as they wanted while women simply maintained their weight when they had adequate sleep – regardless of how much food was available.
But both men and women gained weight when they were only allowed to sleep for up to five hours.
Professor Wright is now working on a new study to better determine the implications of when – not only what – people are eating.
Previous research has linked sleep deprivation with weight gain and also shown how disrupted sleep interferes with levels of stress and hunger-related hormones during waking hours.
A number of studies have observed people who sleep five hours or less are more prone to weight gain and weight-related diseases such as type-2 diabetes.