Lack of sleep has long been linked to obesity, but a new study suggests late-night snacking may not be the primary culprit. The latest findings provide the most compelling evidence to date that disrupted sleep alters the metabolism and boosts the body's ability to store fat.
The findings add to mounting scientific evidence on how disrupted sleep influences the usual rhythms of the body clock, raising the risk of a wide range of health problems from heart disease to diabetes.
Dr Jonathan Cedernaes, a circadian researcher at Uppsala University in Sweden and the paper's first author, said the findings pointed to "the irreplaceable function" that sleep has. "Sleep is not just to conserve energy, it has so many functions," he said.
Time and again, research has linked shift work and lack of sleep to the risk of obesity and diabetes, but the reasons behind this association are complex and have been difficult to elucidate. Insufficient sleep appears to disrupt hormones that control appetite and feelings of fullness.
Those who sleep less have more time to eat, may be too tired to exercise and have less self-control when it comes to resisting the temptation of unhealthy snacks.
A previous study by Dr Cedernaes and colleagues showed that even a short period of sleep deprivation led people to eat more and opt for higher-calorie food.
To complicate matters further, obesity increases the risk of sleep apnoea, a breathing problem that itself disturbs sleep quality.
The latest study provides new evidence of sleep deprivation having a direct influence on basic metabolism and the body's balance between fat and muscle mass.
In the study, published in the journal Science Advances, 15 healthy volunteers each attended a testing session on two occasions, once after a normal night's sleep and once after staying up all night. During the visit, they gave samples of fat and muscle tissue and blood.
After sleep deprivation, people's fat tissue showed changes in gene activity that are linked to cells increasing their tendency to absorb lipids and also to proliferate.
By contrast, in muscle, the scientists saw reduced levels of structural proteins, which are the building blocks the body requires to maintain and build muscle mass.
Previous epidemiological studies also found that shift workers and those who sleep less have lower muscle mass. This may be in part down to lifestyle factors, but the latest study shows that there are also fundamental biological mechanisms at play.
"Sleep loss by itself is reducing proteins that are the key components of muscle," said Dr Cedernaes, although he added that it is possible that diet and exercise could counteract these changes.
The study also found an increase in inflammation in the body after sleep deprivation, which is a known risk factor for Type 2 diabetes.
However, the authors said it would be important to investigate further to see whether the short-term changes they identified were sustained in people working shift patterns or experiencing sleep deprivation over longer time periods.
The link between sleep deprivation and illness is of growing concern due to the increase in shift work and changes in sleep patterns across the world.
Last year, a review of 28 existing studies found that permanent night-shift workers were 29 per cent more likely to develop obesity or become overweight than rotating shift workers.
The number of people regularly working nights in the United Kingdom has risen by 260,000 in the past five years, according to the Trades Union Congress, which estimated last year that Britain's late-night workforce had reached 3.2 million - equivalent to one in eight workers.