Scientists have found evidence that a lack of sleep causes changes in brain activity that lead to people feeling hungrier and craving more fattening foods.
Researchers have long pointed to a correlation between a steep rise in obesity in industrialised nations and less sleep.
A causal link was suspected, but science has not been able to explain the mechanism, until now.
A University of California team used MRI (magnetic resonance imaging) scans to spot changes in the brain activity of sleep-deprived test subjects.
''These findings provide an explanatory brain mechanism by which insufficient sleep may lead to the development/maintenance of obesity,'' they wrote in the journal Nature Communications.
Twenty-three participants had their heads scanned twice; once after a full night of sleep and once after being deprived their shut-eye for a night - their brain activity measured the next day as they selected items and portion sizes from pictures of 80 different food types.
Among the tired individuals, the researchers noted impaired activity in regions of the cortex that evaluate appetite and satiation. Simultaneously, there was a boost in areas associated with craving.
''An additionally interesting finding was that high calorie foods became more desirable to the sleep deprived participants,'' said study co-author Matthew Walker of the psychology department at the University of California in Berkeley.
''These findings of impaired brain activity in regions that control good judgment and decision-making together with amplified activity in more reward-related brain regions fit well with … the link between sleep loss, weight gain and obesity,'' he said.
The World Health Organisation says more than a third of adults were overweight in 2008.