Weight loss is a slog, most of us assume. We think of joyless diets, interspersed with exhausting bouts of exercise. And sometimes weight loss can feel like that. But there’s one relatively painless factor that often goes overlooked: the role of good sleep.

In recent years, sleep scientists have become increasingly interested in the idea that going to bed a little earlier (or, if possible, getting up a little later) can help you manage your weight. Now, a study led by Professor Esra Tasali at the University of Chicago Medical Centre found that going to bed 75 minutes earlier each night helps you consume 270 fewer calories each day – the equivalent of three biscuits.

Tiredness doesn’t just make you hungrier – it also means the food you eat will make you fatter than if you ate the same food while you were well-restedCredit:iStock

Researchers recruited 80 overweight volunteers (41 men and 39 women). All routinely slept fewer than 6.5 hours per night. The volunteers were each given personalised “sleep hygiene” counselling sessions, helping them to increase their sleep by an average of one hour and 12 minutes. Those who slept longer were found to have a “significant decrease in energy intake”, according to the results, published in JAMA Internal Medicine. Some volunteers consumed 500 fewer calories each day.

The results make perfect biological sense. When we’re sleep-deprived, our body produces more ghrelin (the hunger hormone) and less leptin (a hormone that makes you feel full). This probably stems from evolutionary science. In the prehistoric era, tiredness was a signal to our brains that we were in danger. Our sleep-deprived bodies were incentivised to pile on calories, in case we needed to sprint away from a sabretooth tiger.

Indeed, we see this hunger dynamic playing out inside the laboratory. In 2016, 14 healthy men and women in their twenties were monitored over two separate four-day stays in Chicago. In the first stay they spent 8.5 hours in bed each night; in the second it was just 4.5 hours. After the fourth night of poor sleep, the volunteers were offered biscuits, chocolate and crisps.

Despite having just eaten a large meal, they couldn’t resist. The volunteers chose food with 50 per cent more calories, including twice the amount of fat, compared to when they were well-slept.

The researchers were particularly interested in a chemical called 2-arachidonoylglycerol (2-AG). When volunteers were sleep-deprived, their 2-AG levels rose by a third, peaking 90 minutes later in the afternoon, and remaining elevated until 9pm. Lead researcher Erin Hanlon concluded that 2-AG may be acting as a chemical signal “increas[ing] the hedonic aspect of food intake, the pleasure and satisfaction gained from eating.”

A sleep-deprived body also struggles to remove glucose from your circulating blood, studies show.

In other words, tiredness doesn’t just make you hungrier – it also means the food you eat will make you fatter than if you ate the same food while you were well-rested. And it boosts your happiness when you eventually get your hands on that snack.

But what can be done about it? My first advice is obvious: get more sleep.

Doctors advise at least eight hours each night, but this is just an average. Some sleepers need more, others less. If you feel sluggish every morning and crave caffeinated drinks, that’s a sign you’re not getting enough sleep.

Our metabolism works differently depending on the time. When we’re awake, our body burns calories. But when we sleep, it moves into calorie saving-mode – a hibernation strategy.

Weekend lie-ins can help slightly, allowing you to repay some of the sleep debt you’ve accumulated over the week. But it’s important not to sleep too long on any one day; this will knock your sleep cycle out of shape.

You can also try moving your food intake earlier in the day. Our metabolism works differently depending on the time. When we’re awake, our body burns calories. But when we sleep, it moves into calorie saving-mode – a hibernation strategy.

Ideally, we would eat most of our calories over breakfast and lunch, followed by a small, easy-to-digest dinner, containing high protein, high fibre, low fat and low sugar.

Before industrialisation, this is indeed how most humans ate. (In the north of England, many people still call lunch “dinner” because for a long time it was the day’s main meal.)

But then work schedules changed, and the spread of electric lights allowed more socialising in the evening. Now, office workers tend to eat a miniscule breakfast, with a quick sandwich at their desk over lunch. Then comes dinner, when they consume most of their calories. This is exactly the wrong way of doing things.

Exercise is more complicated. We tend to burn more stored calories when exercising in the morning, meaning a pre-breakfast jog should be perfect. But we also exercise longer and harder in the afternoon and early evening, because our core body temperature rises throughout the day and peaks in the afternoon, increasing muscle power. It’s a balancing act; you have to find the exercise regime that works for you.

Alcohol is often used as a sedative to help you fall asleep. But that won’t provide good, high-quality biological sleep. Increasingly, young people avoid alcohol entirely. It may well be a good example to follow.

As told to Luke Mintz. Professor Russell Foster is director of the Sleep and Circadian Neuroscience Institute, University of Oxford.

The Telegraph, London

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