The two epidemics in America which we seem to hear about the most are a lack of sleep and widespread obesity. Unfortunately, it’s not just attention-grabbing headlines: according to studies, over one-third of Americans are obese, and one-third don’t get enough sleep.
The closeness of these two figures may be more than a coincidence. A growing body of research shows a strong association between sleep deprivation and weight gain. In a meta-analysis that encompassed 634,511 subjects, both male and female, ranging in age from 2 to 102, researchers found a consistent increased risk of obesity among those who don’t sleep enough.
So what role does sleep loss play in weight gain—and, on the flip side, can quality sleep help with weight loss?
Before a bunch of unhealthy food can cause weight gain, you first have to make the decision to eat that unhealthy food. And there’s a great deal of evidence that sleep plays a major role in deciding whether or not you indulge.
Sleep deprivation dulls activity in the brain’s frontal lobe, which is the region responsible for decision-making and impulse control. So when a coworker offers you a donut, you take it, rather than eating the yogurt you so dutifully packed.
What’s more, being overly tired actually makes your brain more interested in junk than you’d normally be. This is because sleep deprivation lights up your brain’s reward center, leading you to seek “pleasurable, rewarding experiences"—such as the nefarious donut mentioned earlier.
In one study from Berkeley, participants rated the desirability of certain foods both when they were well-rested and then again after sleep deprivation. In the state of sleep debt, the amygdala portion of the brain (which is involved in emotions, pleasure, and appetite, and is an important part of the brain’s reward system) was highly activated. Participants consistently rated unhealthy, high-calorie foods as more desirable than they had when they were well-rested.
The research bears out in real life, too. Sleep-deprived Japanese factory workers are more likely to snack between meals, eat out, and not eat vegetables; Americans who don’t sleep enough consume more sugar and have less variety in their diet; in Germany, inadequate sleep is associated with increased fast food consumption.
And to top it all off, sleep-deprived people also eat bigger portions. Bottom line: sleep helps you resist temptation and make smarter food choices.
Fatigue and fullness
So say you’re sleep-deprived and you splurge on two (okay, three) slices of pizza at lunch. At least you’ll be full for a while and not eat any more waistline-expanding goodies, right?
Well, maybe not.
Short sleep disrupts the balance of your hormones, including leptin and ghrelin. Leptin is often referred to as the “satiety hormone,” causing you to feel full and suppressing appetite, while ghrelin triggers hunger and plays a large role in initiating eating.
When you’re not well-rested, your leptin levels plummet and your ghrelin levels rise; one study found that subjects who slept for 5 hours had 15.5% lower leptin than those who slept a full 8 hours, and 14.9% higher ghrelin. This means that you’ll not only be eating less healthy, more caloric food—you’ll also feel hungrier and seek food more frequently.
Sleepless and stress-full
Among the many benefits of proper sleep is that it can reduce stress, which, in turn, can help reduce your weight. How? It comes back to another hormone—this time, cortisol.
Cortisol is a hormone that is released in response to stress, and its levels are closely tied to our natural sleep/wake cycles. So when those cycles get disrupted, so do those levels, causing a spike in cortisol in the bloodstream.
This spike doesn’t just make you feel stressed out. Cortisol causes fat to be stored around the organs (especially visceral organs, which translates to belly fat), and also causes fat cells to become larger. Studies have shown that elevated cortisol can cause increased belly fat even in otherwise slender individuals.
You know how not sleeping well makes you feel groggy and lethargic? Well, turns out your metabolism feels pretty much the same way.
When you’re well-rested, your metabolism is a well-oiled machine, efficiently processing the calories that you consume. On the flip side, when you’re in a state of sleep deprivation, your groggy metabolism can’t keep up with your food intake. What causes this breakdown? It all comes down to insulin.
See, insulin plays an important role in helping our body convert sugar into energy for our cells. When our body can’t properly use insulin (insulin resistance) that sugar remains in our bloodstream and eventually is converted into fat. This is the case for those who have diabetes—and, research shows, for those who aren’t getting enough sleep.
One study showed that after just four nights of short sleep, subjects’ ability to respond to insulin decreased by 16%—a difference comparable to that between the cells of obese vs. lean people—and the insulin sensitivity of their fat cells dropped by 30%. The latter is particularly important because fat cells play a crucial role in storing and releasing energy. Meanwhile, insulin resistance in the brain means that insulin can’t do its job of reducing hunger cues.
One report put it in stark terms: “Chronic sleep loss can reduce the capacity of even young adults to perform basic metabolic functions, such as processing and storing carbohydrates or regulating hormone secretion.”
As if your metabolism wasn’t getting a big enough blow from the insulin resistance, there’s this: sleep deprivation reduces the production of thyroid-stimulating hormone, which is an essential player in proper metabolism. Ouch.
Too sleepy to sweat
Anyone who has tried to slim down or get into better shape knows the importance of regular exercise, as well as how tough it can be to get into a workout routine. To the surprise of exactly nobody, not getting enough sleep makes it much more difficult to achieve this.
It’s intuitive—when you’re tired, you don’t want to go exert a bunch of energy. And studies show that subjects with sleep problems report a significant reduction in their levels of physical activity. What’s more, the increased ghrelin and decreased leptin levels associated with sleep loss mean an overall reduction in energy expenditure.
And if you do drag your tired butt to the gym, you’ll be fighting an uphill battle to keep yourself there for a full workout. Sleep deprivation increases your perceived exertion and increases the likelihood that you’ll cut your sweat session short.
Sweet dreams for good genes
Your lifestyle is a huge factor in determining your weight, but the fact is that genetics also play a role. This can be super discouraging to those who are working hard to eat right and work out but still can’t lose the weight because of a genetic predisposition to a higher BMI.
However, research shows that adequate sleep can reduce the influence your genetics have on your weight. In a study of identical twins that looked at BMI, genetics, and lifestyle factors including diet, exercise, and sleep habits, they found that the BMI variations in those who slept adequately were less dependent on genetics. However, those who slept less saw 70% of their BMI variations come from hereditary factors.
The final word
From the food you choose to eat, to how your body processes that food, to your workouts, to the relative impact of all of these components, sleep influences every aspect of your weight. Diet and exercise may get more press, but science has made it increasingly clear that sleep is the essential third pillar of fitness.
So if losing weight, getting fit, or just maintaining a healthy BMI is one of your resolutions, getting enough shut-eye needs to be as well.