Do you eat more when you’re tired? Your hunger isn’t imagined. A number of physical factors conspire to expand your waistline when sleep gets short, says Professor Matthew Walker
When it’s whenever of sleep, your own body will cause you to put on weight, due to a number of factors. The first concerns two hormones called leptin and ghrelin. Leptin tells your brain you feel full, so when levels are high, your appetite is blunted and you don’t want to eat. Ghrelin, in contrast, ramps up feelings of hunger, so when levels are high, you feel hungry and want to eat more. Generally, there’s a healthy balance between these two hormones and the opposing forces of hunger and food satisfaction they control. A lack of sleep distorts this balance, though. Take a group of healthy, fit individuals and limit them to four to six hours of sleep a night for several nights (as countless research studies have done) and you see a striking rise in levels of ghrelin, leaving participants constantly feeling hungry. At the same time, levels of leptin decrease, unleashing an extraordinary appetite. What follows is perhaps all too predictable. Participants in these studies will eat 300–500 more calories at each meal than they do when getting eight hours’ sleep. Worse, despite eating more, they don’t feel satisfied by the food. It’s the appetite equivalent of all accelerator pedal and no brake. Add up those extra calories day after day, year after year, and you can see the potential for weight gain and obesity.
Those cravings go unchecked The second factor is poor food choices. When tired, we eat more foods known to accelerate weight gain and associated diseases, such as diabetes. Cravings for sugary sweets (such as biscuits, chocolate and ice cream), heavy-hitting carb-rich foods (such as bread and pasta) and salty snacks (including crisps and pretzels) all increase by more than 30%, compared with food choices we make when getting a solid eight hours. Why do we lust after these foods? It seems we physically ‘switch off’ our better judgement when tired. With my research team at the University of California, Berkeley, I conducted a study in which we scanned people’s brains while they were viewing food items, and then rated how much they desired each one. We did this twice within the same individuals: once when they’d had a full night of sleep, and once after they’d been sleep deprived for a night. We found that rational control regions at the front of the brain, which normally keep our hedonistic food desires in check, had shut down. Worse still, the more primal, deep-brain structures that drive impulsive decisions were revved up in response to desirable food images. Without sleep, we therefore shift to a more primitive pattern of brain activity that favours uncontrolled impulsivity, making us reach for another slice of pizza rather than leafy greens.
We lose muscle, not fat Skimping on your sleep gets in the way of your weight-loss efforts, too. However carefully you cut down on calories, if you sleep just five to six hours a night, 70% of the weight you lose will come from lean body mass (such as muscle), and not fat. Basically, your body becomes especially stingy in giving up fat when it’s underslept, so dieting is ineffective. Obesity isn’t caused by lack of sleep alone, of course, but research shows the global sleep-loss epidemic is a key contributor to the obesity epidemic. Tragically, we’re now observing these effects very early in life. Three-year-olds sleeping just 10.5 hours or less have a 45% increased risk of being obese by age seven than those who get 12 hours of sleep a night. To set our children on a pathway of ill health this early in life because of sleep neglect is a travesty. Prioritising and relishing sufficient sleep is one of the most powerful ways to regain control of our weight and our waistline. Sleep is a path back to a healthy body and longer lifespan.