Lack of sleep makes us eat more, scientists in America have discovered.
It's known that people who sleep for longer are paradoxically less likely to gain weight, but why and how this occurs, and to whom, wasn't understood.
Now, University of Colorado scientist Rachel Markwald and her colleagues have subjected a group of 16 healthy adults, half of them female, to a sleep deprivation trial in which the subjects endured the equivalent of a working week (5 days) with only 5 hours of sleep per night. The same subjects were also studied for a further 5 day period during which they were permitted to take up to 9 hours sleep per night.
The idea was to compare the energy expenditure, hormone levels, food intake and weight of each of the participants when they were well rested (getting 9 hours per night) or had restricted sleep (the 5 hours per night stretch).
Sleep-deprived subjects, the team found, all gained weight and on average consumed at least 6% more calories compared with when they were well rested.
Intriguingly, in this condition the subjects also ate less at breakfast but compensated with a massive 42% higher intake in the evenings; in fact, their post-dinner intake contained more calories than any of their other individual daytime meals.
Part of the increase in food intake, the team found, was to compensate for the extra energy cost associated with being awake for longer. But the increased intake actually exceeded the amount of replacement energy needed, which is why the subjects gained weight.
Women were more vulnerable to the sleep deprivation weight gain than the male subjects.
When they were allowed 9 hours of sleep, the female subjects showed little change in weight, despite having food on tap. The team put this down to self-control. But in the tired state, the subjects lost their inhibitions and binged with the best of them, leading to a significant weight gain.
Weight gain in the subjects produced corresponding changes in hormones, including one called leptin, that signal satiety and rising body fat mass to the brain.
That the brain ignored these stop-eating signals, say the scientists, suggests that a central mechanism must be kicking in to override normal controls and promote feeding to compensate for the metabolic cost of burning the midnight oil.
Publishing in PNAS, the team ask whether, on the basis of their results, sleep should be considered a modifiable risk factor that can assist in weight loss programmes.