Why poor sleep leads to weight gain, and sleep habits while losing weight

Sometimes the scale will not move even though you are sticking to your diet and not missing workouts. That is why the book I co-wrote, Eat to Succeed (published in Korean, 2026), gives a whole chapter to sleep after the ones on eating and exercise. As the book puts it, however well you manage diet and exercise, leave one thing out and the results may not change. That one thing is sleep.

When the book sorts hunger into six kinds, one of them is fatigue hunger: on days you are short of sleep, you reach for sugar and fatty food because they turn into energy quickly. The book says, "What the body really needs at that moment is not calories but deep sleep and rest."

After a bad night, you can feel peckish again soon after lunch. That is because two hormones that control appetite are thrown off at once. Ghrelin is made in the stomach and tells the brain you are hungry. Normally it rises when you have not eaten and falls after a meal. Without enough sleep that control breaks down, the body wrongly decides it is short of energy, and it keeps telling you to eat. More ghrelin also means a stronger pull towards high-calorie food such as pizza, fried chicken and crisps.

Leptin comes from fat cells and tells the brain "that is enough". When sleep is short, leptin drops and the brain does not get the full signal. That is why something can still feel missing after dinner and you keep reaching for more.

One study measured these changes directly. Twelve healthy men in their early twenties had their food and activity held the same while they spent two days with less sleep and two days with more. Over the two short-sleep days, leptin was 18% lower on average and ghrelin 28% higher. Hunger rose by 24%, and the desire for calorie-dense, high-carbohydrate food went up by 33% to 45%.

The study followed only 12 young men for two days at a time, and the researchers themselves list the small number and the lack of energy expenditure measurements as limitations.

Some days you head out determined to have a salad and give in the moment you see pizza. Without enough sleep, the frontal lobe, which handles reasoning and judgement, slows down, while the amygdala and the reward centre, which handle instinct and emotion, become overactive. Faced with high-calorie food in particular, the reward centre reacts strongly and the body moves before the resolution in your head can catch up.

In the book I wrote: "Not being able to resist food is not a matter of weak willpower. It is because a lack of sleep has the brain working abnormally." If you slept badly last night, today's wobble on your diet is not simply a failure of will.

Once people decide to lose weight, they sometimes get up early to fit in a morning workout and stay up late to finish work. One study looked at what happens when you cut sleep while cutting calories. Ten adults with overweight followed the same reduced-calorie diet for 14 days twice. One time they had 8.5 hours a night in which to sleep, the other time 5.5 hours.

They lost about the same weight in both periods. What differed was what they lost. With 8.5 hours, they lost 1.4 kg of fat; with 5.5 hours, only 0.6 kg. Fat-free mass, the part of the body that is not fat, such as muscle and water, went the other way, from 1.5 kg to 2.4 kg, and they were hungrier in the short-sleep period.

Growth hormone, released at night during deep sleep, helps build muscle and burn fat. When deep sleep shrinks because you are short of sleep, very little growth hormone is released. Insulin sensitivity also falls, so higher insulin blocks fat breakdown, and the stress hormone cortisol stays high into the night, gathering fat around the belly and organs and reducing muscle.

That said, it was a small study of ten people over two weeks each, and the researchers list the length and the sample size as limitations. It did not test whether more sleep makes you lose weight. It compared what you lose when you sleep less while dieting.

The same number on the scale can hide different losses. The book also says there is no need to fixate on a single number. A body composition test such as InBody splits your weight into fat and muscle, and Yoonsujung Clinic measures InBody in the clinic during weight consultations.

If you only get up on weekdays because the alarm goes off, start by finding out whether you are sleeping enough for your body. The method in the book is simple. At the weekend, sleep without an alarm until you wake on your own. How long you slept and how refreshed you feel then tells you how much sleep you need.

The book sets seven to eight hours of deep sleep a night as the aim. Quality matters as much as hours. Eight hours in bed is no use if you keep waking and never sleep deeply, and six hours of unbroken deep sleep is better.

Some nights you go to bed early to get more sleep and it simply will not come. So the book covers daytime habits as well as the hour or two before bed. In time order, it looks like this.

On 3.6.9 the 9 pm snack is only for when you are hungry, and that ties in with sleep too. It is hard to fall asleep either very hungry or very full. If you feel hungry before bed, a banana, a glass of warm milk or a few nuts is enough.

When night after night you toss and turn for an hour or two, or wake before dawn and cannot get back to sleep, sleeping pills start to look tempting. Insomnia comes in types: trouble falling asleep, waking often, and waking too early. In the book I put cognitive behavioural therapy for insomnia (CBT-I) ahead of medication. It works by changing mistaken beliefs and habits around sleep.

• Sleep restrictionif you lie in bed for eight hours but only sleep five, you stay in bed for five. You feel more tired at first, but that tiredness helps you sleep better the next night. Once you are asleep for more than 85% of the time in bed, you lengthen it little by little.
• Stimulus controlthe bed is only for sleep. If you have not fallen asleep within 20 minutes, get up, go to another room and come back when you are very sleepy. Repeat this and the brain learns again that the bed is for sleeping.

Sleeping pills can help in the short term, but they do not fix the cause. They can lead to tolerance or dependence and can reduce deep sleep and REM sleep, which lowers the quality of sleep. If you need them, talk to a doctor, take them for a short time only, and always combine them with CBT-I or changes to your routine. The end point is falling into deep sleep on your own without medication. It is the same thinking as in the book's chapter on diet pills, where I wrote that medication is only a tool.

Sometimes you feel you slept well, but the person next to you worries that your breathing seems to stop. If you snore heavily, hear that from others and struggle to stay awake in the day, you should suspect sleep apnoea. Each time breathing stops, the brain wakes without you knowing, so you never reach deep sleep. A sleep study (polysomnography) can confirm it.

Sleep apnoea and weight are tied together. As weight goes up, fat around the neck narrows the airway and blocks breathing, stress hormones rise and metabolism is thrown off, and you gain more weight. That is why mild sleep apnoea can improve with weight loss alone. Lying on your back makes it easier for the tongue and soft palate to block the airway, so sleeping on your side also helps. CPAP, the machine used as treatment, blows air through a mask while you sleep to keep the airway open. At first it can feel stuffy and the noise can be annoying, so it takes time to get used to.

If every night your legs itch, tingle or feel as if something is crawling on them, so you cannot stop moving them, it may be restless legs syndrome. Low iron or magnesium is often named as a cause, so get it checked with a test, and avoid caffeine and alcohol, which make it worse.

The book also says that if there is a medical problem, you should get expert help. In these cases there is no need to rely on habits alone.