Why a lack of sleep often makes you crave late-night snacks

Sweet—and salty—dreams.
Sweet—and salty—dreams.
Image: Reuters/Yuya Shino
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Decades of studies have shown that those who sleep poorly are more likely to be obese—our need for sleep and calories seem deeply linked. Why that may be, however, has been difficult to understand.

A new study published in the journal Sleep, suggest that the reason so many of us crave late-night snacks may have to do with how sleep changes brain chemistry. Though it’s a small study, it builds on previous research about how the body responds differently to food consumption at various times of the day.

Researchers from the University of Chicago recruited 14 men and women in their twenties to be monitored in a sleep lab and split them into two groups. In two, four-day sessions, both groups’ food consumption and sleep time were strictly controlled. They all received three meals at 9am, 2pm, and 7pm. During the first visit, one group slept 7.5 hours a night and the other slept a little more than four hours. Then, during the next visit, they kept their meal times the same but swapped their sleeping hours.

After the final night in each session, the volunteers were offered as much food as they would like to eat. In both sessions, the researchers found that those who had slept less ate, on average, 300 calories more than those who had slept more. That is, they ate much more than would be needed in energy terms to compensate for three fewer hours of sleep.

To understand why, the researchers looked at the levels of chemicals in the blood in each group. They found that, apart from higher levels of ghrelin (which boosts appetite) and lower levels of leptin (which signals satiety), those who slept less had higher levels of an endocannabinoid.

“We know that marijuana activates the endocannabinoid system and causes people to overeat when they are not hungry, and they normally eat yummy sweet and fatty foods,” Hanlon told the Guardian. “Sleep restriction may cause overeating by acting in the same manner.”

They also found that levels of the endocannabinoid peaked at about 2pm the next day. That means that people who sleep poorly may have food cravings for a sustained period of time, which increases their risk of becoming obese. This should serve as a warning, especially when combined with previous research that shows how the circadian rhythm makes people crave sweet, starchy, and fatty foods more in the evenings.

The study’s sample size is small and it will need to be reproduced among many more volunteers to be sure. Still, it is the first time that we’ve got a hint about why a lack of sleep makes us fall for the guilty pleasure of late-night snacks.