There's lots of evidence that getting too little sleep is associated with overeating and an increased body weight.
The question is, why? Part of the answer seems to be that skimping on sleep can disrupt our circadian rhythms. Lack of sleep can also alter hunger and satiety hormones.
Now, a new study finds evidence that sleep deprivation (getting less than five hours of sleep per night) produces higher peaks of a lipid in our bloodstream known as an endocannabinoid that may make eating more pleasurable.
So, what's an endocannabinoid? If you look at the word closely, you may already have a clue. The prefix endo means inner, or within. And cannabinoid looks like ... you got it: cannabis.
Our bodies produce compounds that seem to act on the same parts of the brain as marijuana does. We've all heard of the marijuana munchies, right?
The new study, based on blood samples, documents a novel finding: The daily rhythm of a particular endocannabinoid, known as 2-AG, is altered by a lack of sleep.
And these changes "could be driving intake for more palatable foods," Erin Hanlon, a neuroscientist at the University of Chicago Medical Center, told us. She's an author of the new study, published in the journal Sleep.
"We found that sleep restriction boosts a signal that may increase the hedonic aspect of food intake," says Hanlon. In other words, being sleep deprived may produce a stronger desire to eat.
To study how sleep influences appetite and eating, Hanlon and her colleagues recruited 14 healthy, young adults to take part in an experiment at the university's sleep lab.
"These were all people who were ... normal sleepers," says Hanlon. They typically slept about eight hours a night.
The study was divided into two parts, each lasting four days. For one session, the participants were allowed to follow a normal sleep schedule, about 8 1/2 hours per night.
But during the other session, they agreed to a crazy schedule. They went to bed at 1 a.m. and were woken up at 5:30 a.m., so that they got a maximum of just 4.5 hours of sleep per night.
In both sessions, study participants were offered buffet-style meals and plenty of snacks, including candy and chips.
"They were given way more food than they could ever eat, "says Hanlon.
It turned out that when participants were sleep deprived, they ate about 400 more calories from snacks. That's "a lot more," Hanlon says.
Generally, circulating levels of 2-AG rise slowly during the day and peak in early afternoon. But this study found that when people were sleep deprived, their 2-AG levels peaked higher and stayed elevated longer.
And that peak "corresponds to the same time of day when [the participants] said they were feeling hungrier, or their desire to eat was stronger," Hanlon told us.
Frank Scheer, a chronobiologist at Harvard Medical School, wrote a commentary on the new study that also appears in the journal Sleep.
"The idea is that when the levels of endocannabinoids are higher, food [is] more appealing," Scheer explains.
He says this paper makes an important contribution. "We did not know whether there would be an effect of sleep restriction on endocannabinoids, so this is really the first study to nicely show that," Scheer says.
Scheer says the new findings fit with brain-imaging studies of sleep-deprived people. These studies have found "that the brain areas involved in reward show increased activity when people [see] images of food" — especially salty, sugary and fatty snacks.
There's still a lot to learn about all the ways that sleep can influence our appetites and eating behaviors, but these findings serve as a reminder of the importance of getting a good night's sleep.