Not Getting Enough Sleep Makes it a Lot Harder to Get in Shape

Most people can't go long without seeing a reminder that they probably need more sleep. Other than leaving you grumpy and fatigued, however, there's at least one side-effect of cutting back on shuteye that we often fail to consider: Lack of sleep can also dramatically sabotage your best efforts to get in shape.

Among other things, sleep deprivation has been shown to increase the loss of muscle when you go on a diet. In one study, a team of US researchers got a group of overweight men and women to stay in a sleep research laboratory while they dieted for two weeks.

During the first phase of the study, test subjects were deprived of sleep, registering a little over five hours each night. In the second phase, they ramped up the amount of time spent in bed, clocking up around 7.5 hours of sleep per night. The result? Sleep deprivation didn’t affect the amount of weight lost—dieters lost around 6.5 pounds in both conditions. It was a different story, however, when the researchers analyzed where the lost weight came from.

During the weeks with adequate sleep, roughly half of the lost weight came from fat. But in the sleep deprived condition, four out of every five pounds lost came from lean body mass—muscle, not fat. In other words, overweight adults on a diet lost 60 percent more muscle when they were deprived of approximately two hours of sleep each night.

Granted, this was a short trial, where each phase of the study lasted just 14 days. And nobody taking part lifted weights or ate enough protein, two things that we know help you retain muscle mass during a diet. The research, however, does hint at the possibility that when you are not getting enough sleep, the body is reluctant to give up its fat stores. Muscle mass is depleted while fat is retained.

It gets worse: You may have noticed that you feel like you want to eat more when you’re tired. That’s because lack of sleep can mess with some of the hormones involved in both hunger (which refers to the physical need for food) and appetite (the desire for food). Studies show that skipping sleep leads to elevated levels of a hormone, known as ghrelin, that makes you hungry. It also suppresses a counterpart hormone called leptin, which plays a role in regulating the amount of food you eat. The unsurprising result is that you eat more. In some trials, subjects deprived of sleep would end up munching their way through the best part of 600 additional calories per day.

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Among men, sleep deprivation isn’t great news for testosterone levels, either. A team of researchers at the University of Chicago took a group of lean, healthy young guys in their mid-twenties and restricted them to just five hours of sleep a night. A week later, blood tests showed a 10 to 15 percent drop in testosterone. Cutting down on sleep—even for just seven days—can leave you with the testosterone levels of a man at least a decade your senior. If you’re a man, low testosterone levels will, in turn, often leave you tired and fatigued. Testosterone can sharpen your ability to focus, and when levels are low, it’s harder to concentrate. Your sex drive could take a dip, too.

The silver lining here is that for sleep to affect your gains, specifically, it needs to be a pattern: A single night of missed sleep is unlikely to do any damage. In one trial, researchers from Midwestern State University found that going an entire night without sleep had no effect on training performance in a group of collegiate weightlifters.

Although total mood disturbance was increased by sleep deprivation, the subjects were able to lift just as much as weight as they did following a normal night of sleep.

If you’re regularly missing out on precious hours of rest, however, getting a little extra sleep each night can have an almost “drug-like” effect on your performance. Most research has looked at how cutting sleep affects an athlete's ability to complete various tasks designed to simulate competition. Rather than focus on the effects of sleep deprivation, however, scientists from Stanford University asked a different question: Could extra sleep give athletes an edge over their competition?

To find out, they looked at the effect of sleep extension on sprinting speed and shooting accuracy in members of the men’s varsity basketball team. For the first two weeks of the study, the men kept to their normal schedules—which involved sleeping for a little over 6.5 hours a night—while the researchers measured how fast they could run and how accurate their shots were.

During phase two, the players were told to get as much sleep as they could, with the goal of spending 10 hours in bed each night. They were also encouraged to nap during the day. The actual amount of time the men spent asleep rose to an average of almost 8.5 hours a night. And this two hours of additional sleep had a big impact on their performance.

By the end of the extra-sleep period, the players had improved their shooting accuracy by nine percent. Sprint times were down, on average, by 0.7 seconds. In fact, every player on the team was faster than they were at the start of the study. Not only that, their mood improved: They reported feeling less tired and more energetic. To quote the research team directly:

Sleep extension, in other words, can have drug-like effects on athletic performance, and yet is completely safe. In fact, not only is it completely safe, studies also show a tight link between sleep duration and risk of heart disease, diabetes and dementia.

When you lose weight, more of that lost weight will come from fat rather than muscle. You won’t feel as hungry, and you will have fewer food cravings. Even your mood will improve. You can spend a lot of time hunting down the latest breakthroughs in diet and exercise, trying all the latest supplements and experimenting with every cutting-edge “hack” you come across. But if you’re skimping on sleep, you’ve got your priorities all wrong.

Christian Finn is a UK-based personal trainer and exercise scientist. He writes frequently about fitness and nutrition on his personal site, MuscleEvo.

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