How to burn fat overnight

Engage with your circadian rhythms and lose weight while you sleep.

Maria Cross MSc
Oct 12 · 8 min read

Everyone knows that weight loss is about what you eat and what you don’t eat. But there’s another critical element to consider. Losing weight is also about aligning your eating and sleeping habits with your internal biological clock. Find out how to set this clock to maximum fat-burning mode.

Buried deep within your brain is an ancient biological clock, one that functions with extraordinary, near 24-hour precision. Recognising this clock (aka the circadian rhythm) is your weight loss superpower.

Located within the suprachiasmatic nucleus region of the hypothalamus, this biological clock is mysterious, and more than a little otherworldly. It responds to the rising and setting of the sun so that your patterns of sleep and wakefulness match the rotation of the Earth about its axis.

Not only is there a clock in your brain, there are also “peripheral” timekeepers in other parts of your body, including the heart, liver and pancreas, that are synchronized by the master clock in the brain.

Your metabolic rate falls by around 15 per cent during sleep. If you think about it, this is a surprisingly small reduction for such apparent inertia. But don’t be deceived; as you slumber, blissfully unaware of the workings of your autonomic nervous system, your body is busy carrying out innumerable repairs and metabolic processes, and your brain is burning fuel furiously.

As you sleep, your body clock orchestrates the secretion of important metabolic hormones that influence the way you gain and lose weight. These hormones — which include growth hormone, insulin, ghrelin and leptin — are produced in a cyclical manner. Obey your biology, as outlined in the following three steps, and you’ll find that losing weight is much easier when you work with your body, not against it.

Sleep it off — in three synchronized steps

“Disruption of circadian rhythmicity has been implicated in the pathogenesis of several diseases, including metabolic disorder.” (Marcheva et al)

Step 1 — Get the timing right

The evidence is accumulating that timing of meals is an important determinant of how food is burned for energy and is a predictor of body mass index (BMI) in humans. Meal timing can affect not only metabolism but also core body temperature, performance and alertness.

To burn fat, you must be in fasting mode by the time you go to bed. To be in fasting mode, you have to stop eating at least three hours before retiring and avoid late-night snacking.

The reason for this is that under normal conditions your body switches to burning fat overnight as opposed to glucose. This won’t happen if you have a stomach full of food waiting to be digested.

Once you are in fasting mode, and asleep, the first of several important metabolic hormones swings into action. Growth hormone is dominant at night and becomes active at the beginning of the first deep sleep cycle. GH is a fat-mobilising hormone that stimulates the release of fat from your fat stores — the very fat you want to burn.

This fat provides the body with the fuel it needs to perform its many metabolic activities, including those nightly repair jobs carried out by your autonomic nervous system. Lack of GH at this time increases muscle protein breakdown and can result in increased fat mass.

The timing of your first meal of the day is also relevant. Ideally, leave 16 hours between your evening meal and your first meal the following day. This way, you are practising “intermittent fasting”. Although there is no official definition of this increasingly popular practice, the IF fundamentals involve a pattern of eating based on extended periods of abstinence. A long overnight fast is sometimes called ‘time-restricted feeding’ and is a surprisingly easy way to fast, given that you are asleep for half the time.

During the last few hours of a fast — in the morning when you are awake — levels of substances called ketones start to rise. Ketones are made in the liver from fatty acids taken from your body fat and they provide fuel for the brain. Thus, when you are making ketones, you are burning body fat.

There’s nothing new about intermittent fasting. IF is an ancient practice, taking on varying formats, for different motives, and practised by populations all over the globe. It is what our Paleolithic hunter-gatherer ancestors did, in the absence of shops and a food industry hell bent on promoting mindless snacking. Challenges in the form of having to find food meant that pre-agricultural humans had to function well when in a fasted state — they had to make critical decisions and move fast.

Takeaway: Be in fasting mode when you go to bed, to enable growth hormone to start burning fat and wait 16 hours before eating your first meal of the day.

Step 2 — Make sure you get at least 7 hours of uninterrupted sleep

Insufficient sleep raises the risk of developing obesity, and this is true of humans of all ages. Just five sleep-deprived nights can disrupt your appetite and make you eat more than you would ordinarily.

What’s a sleep-deprived night? Numerous studies looking at the link between sleep and obesity have confirmed that less than six hours’ sleep at night is the damage threshold. This damage has been linked with diabetes, metabolic syndrome and obesity.


Sleep deprivation can cause another metabolic hormone to go awry. Insulin is a key factor in fat metabolism. Insulin regulates blood glucose by removing any excess and turning it into fat. Insulin is naturally low at night, when growth hormone is dominant.

If you don’t get enough sleep, your insulin sensitivity decreases, growth hormone is suppressed and insulin remains high, meaning you can’t burn fat. Without enough sleep, insulin sensitivity — your ability to use insulin efficiently — decreases.

After 5 days of sleep restriction, your insulin sensitivity can be 40 per cent lower than normal.

Lack of sleep also disrupts two other important metabolic hormones: leptin and ghrelin. Both are involved in appetite control.

Your goal, if you want to burn fat, is to produce as much leptin as you can. Leptin is an appetite-suppressing hormone, secreted by fat cells, that informs the brain that you are full, and further eating is not required. However, production of this key appetite controller falls if you are sleep-deprived.

Ghrelin, on the other hand, does just the opposite. It is secreted in the stomach and stimulates appetite by telling your brain that you are hungry. Food is required and your brain urges you to eat. This is a hormone that definitely needs reining in.

Sleep too little and the whole metabolic balancing act is thrown into reverse. Without enough sleep you produce more ghrelin than leptin.

Interestingly, sleep-deprived individuals tend to eat more carbohydrates and snack more. The more carby snacks you eat, the more insulin you produce. The more insulin you produce, the more fat gets packed away.

Takeaway: Sleep for at least 7 hours to ensure that your fat burning, appetite-controlling hormones can function at maximum capacity.

Step 3 — Avoid social jetlag

Unfortunately, it’s not just lack of sleep that can lead to weight gain; so too can sleeping at the wrong time. The circadian rhythm is controlled by exposure to light and when this is disrupted you are at increased risk of, among other poor health outcomes, metabolic dysfunction.

When mice, which are nocturnal mammals, are fed during the day (when they would normally be asleep) they gain significantly more weight than mice fed only during the night, when they are normally awake and feeding.

Eating at night changes your metabolism. People with “night eating syndrome” may eat the same amount as day eaters, but they have “abnormal rhythms of metabolic hormones”, including disruption of blood sugar control.

This phenomenon of living against the clock has been termed “social jetlag” by scientists, and this form of jetlag has been found to significantly increase the likelihood of being overweight.


Much of this disruption has been blamed on electric lighting and the 24-hour lifestyle to which we have grown accustomed. We create our own, dissonant settings, which ultimately clash with our clocks and create disharmony in our hormones.

Work and school timetables, and the use of alarm clocks to align wakefulness with those timetables, contribute to the amount of social jetlag accrued by each individual. Social jetlag is at its most extreme during adolescence (they really do need to stay in bed half the morning) but continues throughout the working life.

Daytime sleep has been found to be associated with “marked” elevations of glucose and insulin. This is a tricky one for shift workers, but the fact is that humans are designed to sleep at night, not during the day. Studies consistently show that overweight and obesity are far more prevalent in shift workers than day workers, and that shift work is a risk factor for becoming overweight.

I’m sorry to say that there is no solution to this issue if you are a shift worker; I can only advise that you make sure that you still get seven to eight hours of unbroken sleep, and in a darkened room.

“In a sense, shift workers, who are exposed to high levels of light at night in the workplace, have served as society’s canaries in the coal mine for maladaptive consequences of nighttime light exposure.” (Fonken & Nelson)

Takeaway: Sleep at night, eat during the day

The whole world is getting fatter — prevalence of obesity across the globe has doubled since 1980 — and at the same time, there has been a parallel trend in reduced sleep duration. There is growing and convincing evidence that the two phenomena are linked. Consider the US, where Americans are definitely sleeping less. A 1960 survey conducted by the American Cancer Society found that on average people commonly enjoyed between eight and nine hours sleep a night.

Fast-forward to 1995, when another survey, this time by the National Sleep Foundation, found that sleeping for seven hours was the norm. By 2008, over 30% of adults reported sleeping fewer than 6 hours each night.

Every night, as you sleep, your body balances its energy accounts and in the morning you wake up and discover how the books are looking. Losing weight doesn’t have to be a battle with your body, so learn to work with it, not against it. Check your settings, and make sure you are in fat-burning mode.

Obviously now that you have the fundamentals of your pattern of eating, the next step of your weight-loss programme centres on what to eat. That’s next.

Maria Cross MSc

Written by

Nutritionist and nutrition science writer, specialising in diet and mental health. Subscribe to for free brain food guide. @MariaXCross

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