One five-hour night, and the whole day goes off course. The 10 am coffee is not enough, the pastries at the meeting start calling, dinner comes late and bigger than planned. People tend to think sleep affects weight through these slips. It also affects what you lose when you slim down, and that is the part people know least.
Same diet, three hours less sleep
At the University of Chicago, ten adults with overweight, seven men and three women in their forties, followed the same 14-day low-calorie diet twice, in the lab. Once with 8.5 hours in bed each night, once with 5.5 hours. The calories were identical, only sleep changed (Nedeltcheva et al., 2010).
Sleeping 8.5 hours, they lost 1.4 kg of fat. Sleeping 5.5 hours, only 0.6 kg, which is 55% less. Lean mass went the other way, 1.5 kg lost with the long nights, 2.4 kg with the short ones, which is 60% more. With less sleep, they were also hungrier and burned less fat.
All of this in two weeks, on a diet followed to the letter, without the poor food choices that usually come with short nights. The study is small and short, and its authors say so. It shows that sleep decides part of what you lose, fat or muscle. I mention it in a few lines in the article on weight loss plateaus.
One hour less is enough
In South Carolina, 36 adults with overweight or obesity followed a diet for eight weeks. Twenty-one of them also had to cut their nights by about an hour, five evenings a week, and slept freely on the other two. In total, they slept nearly three hours less per week.
Both groups lost the same amount of weight. But in those who slept less, the share of fat in the weight lost was smaller, and the catch-up nights were not enough to make up for it (Wang et al., 2018).
Why you get hungrier
A few short nights are enough to make your body less sensitive to insulin, raise your hunger hormone and lower your fullness hormone. Again in Chicago, twelve healthy young men spent two shortened nights and two long nights. After the short nights, leptin, which signals fullness, had fallen by 18%, and ghrelin, which pushes you to eat, had risen by 28%. Their hunger had climbed by 24%, and their appetite for calorie-dense, high-carbohydrate food even more (Spiegel et al., 2004).
Your body feels short of energy and looks for the quickest source, often processed food and carbohydrates. A team at King's College London pooled eleven trials. After shortened nights, participants ate on average 385 calories more per day, with more fat and less protein, while their energy use did not change (Al Khatib et al., 2017). Sleeping more does not speed up your metabolism in itself, but lack of sleep makes you eat more and eat worse.
You move less without noticing
Lack of sleep also lowers your NEAT, all the everyday energy use that is not exercise. It takes away the urge to train and pushes you to skip or shorten sessions. In Lübeck, Germany, fifteen men wore an activity tracker after two normal nights, then after two shortened ones. After the short nights, they moved less during the day, and less intensely (Schmid et al., 2009). In that particular study, they did not eat more. From one study to the next, the effect works through your plate, through movement, or both.
One bad night on its own does not change much. Repeated, it adds up. Bad nights again and again also lower your resting metabolic rate.
Muscle is what is really at stake
Lean mass is not only muscle, it also includes water and organs, but muscle is part of it. And muscle explains a very large part of the differences in resting metabolic rate from one person to the next. Losing muscle during a diet lowers that resting metabolic rate even further, and what comes next gets harder. The link with sleep runs both ways. More muscle often goes with better sleep, and more fat with worse sleep. The quality of your weight matters as much as the number on the scales.
Lack of sleep also weighs on your blood sugar, as I explain in the article on prediabetes. And if your question is really about what time you eat dinner, that is a different subject, covered in the article on eating late.
When nights get longer
It works the other way too. In Chicago again, 80 young adults with overweight who slept less than six and a half hours a night were randomised. Half had a single session of advice on lengthening their nights, with no diet at all. They slept about 1 hour 10 minutes more, and ate 270 calories a day less than the control group, at home, in their normal lives. Their weight went down (Tasali et al., 2022).
Managing your body composition with chronically poor sleep is very hard. Improve your nights first before expecting big changes. Regularity matters more than length. Go to bed at a fixed time, get natural light in the morning, cut down on screens in the evening, avoid caffeine in the afternoon and keep your bedroom cool.
After a bad night
Lean on protein all day. It takes up room and curbs the craving for burgers and chips.
Keep activity light. No intervals if you do not have the energy, but three or four ten-minute walks. A sauna or a hot bath can add to it, without replacing exercise.
And if you can, take a nap of 30 to 90 minutes. It can transform the quality of the next ten hours.
Tonight, pick a bedtime and stick to it for seven days in a row, weekends included. The South Carolina trial showed it, lie-ins do not make up for everything.
This article is for information only and does not replace medical advice.