When we get poor sleep, our bodies go into survival mode. This is likely wrought by evolution: Little sleep, in prehistoric days, was probably associated with danger or scarcity.
Scientists now know that sleep deprivation sets off a chain of metabolic responses to ensure our survival: Namely, it makes us hungrier (so we have more energy to face the threat).
In ancient times, this would have helped us stay alive. But in these times of abundant calories, it seems to just propel us to overeat.
A recent meta-analysis of 11 studies finds evidence that, overall, sleep deprivation seems to cause a person to eat an average of 385 extra calories the next day. (For reference, there are about 380 calories in a medium French fries at McDonald's, and generally people should aim to eat around 2,000 to 2,500 calories per day.)
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It's not a ton of extra food, but perhaps enough to start tipping the scale.
"In the long term, this may implicate weigh gain," the study, published in the European Journal of Clinical Nutrition, concludes. Because not only did the participants eat more after being sleep-deprived, they also didn't compensate by expending more energy. The analysis also found some evidence that sleep deprivation leads people to crave calorie-dense fats over protein.
One pound of fat is roughly equivalent to 3,500 calories. So that means it would take around nine days of poor sleep to gain a pound, if the study generalizes to the real world.
All studies included in the meta-analysis had their participants intentionally restrict their sleep to around four hours. The next morning they were carefully monitored, in terms of both what they ate and how much energy they expended. (All the studies also included a control group of people who slept really well — seven to 12 hours — for comparison. There were around 500 participants included across all 11 studies in the analysis. Some of the studies restricted sleep for a few days, others for a few weeks. Most of the studies took place in a lab or hospital.)
But how does poor sleep lead to increased hunger? The researchers here hypothesize that our poorly rested brains may simply get more excited by food. (Other neuroscience researchhas suggested sleep restriction makes our brain regions that respond to food more active.)
Another hypothesis (not as well supported in this analysis) is that poor sleep messes with our hunger hormones. In a separate overnight sleep study of 1,024 individuals, poor sleep was associated with the misregulation of the hunger hormones leptin and ghrelin. The study concluded the hormone imbalances observed "are likely to increase appetite."
Sleep researchers aren't typically worried about the effects of one night of sleep loss on health. They're worried about what happens when poor sleep becomes chronic. And in America, chronic sleeplessness is rampant: The Centers for Disease Control and Preventionestimates that 30 percent of adults regularly get less than six hours a night. (Researchers generally say six hours a night and under is where they start to see negative health consequences.)
Meanwhile, around 70 percent of Americans are obese, and scientists say evidence of a link between the two health crises is mounting.
It's clear that poor sleep is horrible for our health in other ways, too
The meta-analysis is the latest in part of a growing body of literature that finds poor sleep — getting too few hours, sleeping on an inconsistent schedule, or waking up too often during the night — is correlated with higher blood pressure, higher body mass index, andincreased calcification of the coronary artery. (Sleep may play a role in maintaining the wide health gaps between white and black Americans.)
It's also believed that poor sleep sets off a chain of metabolic events that make us more susceptible to diabetes.
In lab experiments, people who slept only five hours night for one week became less sensitive to insulin, which makes it harder to maintain blood sugar levels. One study even looked at individual fat cells of people who had been restricted to 4.5 hours of sleep for four days. "The percentage of insulin resistance [in the cells] was comparable to somebody who has full-on Type 2 diabetes," Josiane Broussard, a co-author on that study who researches sleep at the University of Colorado, told me in May.
In yet another tightly controlled lab study, 24 healthy participants who had their sleep shifted by one hour each day (simulating jet lag) started to look prediabetic after a three-week trial. Their resting metabolic rates dropped 8 percent. "Assuming no changes in activity or food intake," that "would translate into ~12.5 pounds increase in weight over a single year," the study, published in Science Translational Medicine in 2012, concluded.
The caveat to all these studies is that the lab-mandated sleep restriction tends to be intense (participants are generally monitored 24-7 and are forced awake if they doze off). It's possible these studies overexaggerate the effects of poor sleep.
It's also not known if getting better sleep can reverse the signs of diabetes, heart disease, and obesity brought on in part by poor sleep. (Long-term studies on sleep interventions are difficult because the reasons people don't get good sleep can be hard to control: like having to work shifts, living in a noisy neighborhood, or having to take care of young children.)
But if you can get better sleep, why not try? It's at least more pleasant than dieting to maintain weight.