A lack of sleep can make you fat as it triggers the "munchies" lowering self control, according to new research.
The study suggests that losing sleep boosts hunger and makes people more likely to scoff sugary snacks or unhealthy junk food.
Skimping on sleep has previously been associated with overeating, poor food choices and weight gain.
The new study, published in the journal Sleep, shows how a lack of shut-eye starts the process, amplifying and extending blood levels of a chemical signal that enhances the joy of eating, particularly the guilty pleasures gained from sweet or salty, high-fat snacks.
Sleep-deprived participants in this study - 14 young and healthy volunteers - were unable to resist what the researchers called "highly palatable, rewarding snacks" - meaning biscuits, chocolate and crisps - even though they had eaten a meal that supplied 90 per cent of their daily caloric needs just two hours earlier.
The effects of sleep loss on appetite were most powerful in the late afternoon and early evening: times when snacking has been linked to weight gain.
Doctor Erin Hanlon, of the University of Chicago, said: "We found that sleep restriction boosts a signal that may increase the hedonic aspect of food intake, the pleasure and satisfaction gained from eating.
"Sleep restriction seems to augment the endocannabinoid system, the same system targeted by the active ingredient of marijuana, to enhance the desire for food intake."
This chemical signal is the endocannabinoid 2-arachidonoylglycerol (2-AG). Blood levels of 2-AG are typically low overnight. They slowly rise during the day, peaking in the early afternoon.
But when the study subjects were sleep-deprived endocannabinoid levels rose higher and remained elevated through the evening, beyond the typical 12.30pm peak.
During that period, the sleep-restricted volunteers reported higher scores for hunger and stronger desire to eat.
When given access to snacks, they ate nearly twice as much fat as when they had slept for eight hours.
Dr Hanlon said the increase in circulating endocannabinoid levels "could be a mechanism by which recurrent sleep restriction results in excessive food intake, particularly in the form of snacks, despite minimal increases in energy need."
She said: "The energy costs of staying awake a few extra hours seem to be modest.
"One study has reported that each added hour of wakefulness uses about 17 extra calories. That adds up to about 70 calories for the four hours of lost sleep.
"But, given the opportunity, the subjects in this study more than made up for it by bingeing on snacks, taking in more than 300 extra calories. Over time, that can cause significant weight gain."
She added: "If you have a Snickers bar, and you've had enough sleep, you can control your natural response.
"But if you're sleep deprived, your hedonic drive for certain foods gets stronger, and your ability to resist them may be impaired. So you are more likely to eat it. Do that again and again, and you pack on the pounds."
Doctor Frank Scheer, of Harvard University's Brigham and Women's Hospital, said the results support "the novel insight that sleep restriction leads not only to increased caloric intake" but also to "changes in the hedonic aspects of food consumption."