Your body clock, metabolism and digestion interact in complex ways – meaning it’s not just what you eat, but when you eat, that matters.

When young adults start university, they often gain weight.

In the United States, they have a name for this phenomenon: the “freshman 15”, referring to the 15 lbs typically accrued during students’ first year of living away from home.

In part, this weight gain can be explained by the substitution of home-cooked meals for ready meals and fast food, combined with reduction in physical activity.

Increasingly, however, scientists are fingering an additional suspect: circadian disruption, brought about by a culture of late-night eating, drinking, and inconsistent sleep patterns.

For decades, we’ve been told that weight gain, together with associated diseases such as type 2 diabetes and heart disease, are a simple matter of the quantity and type of food we consume, balanced with the number of calories we expend through exercise.

But mounting evidence suggests that timing is also important: it’s not just what you eat, but when you eat that matters.

The idea that our response to food varies at different times of day dates back a long way.

Ancient Chinese medics believed that energy flowed around the body in parallel with the sun’s movements, and that our meals should be timed accordingly:

7-9 am was the time of the stomach, when the biggest meal of the day should be consumed;

9-11 am centered on the pancreas and spleen;

11 am-1 pm was the time of the heart, and so on.

Dinner, they believed, should be a light affair, consumed between 5pm and 7pm, which was when kidney function predominated.

Although the explanation is different, modern science suggests that there is plenty of truth in that ancient wisdom.

Consider studies of dieters. Most weight-loss schemes revolve around reducing the overall number of calories consumed – but what if the timing also determined the benefits?

those who consumed most of their calories at breakfast lost two and a half times more weight than those who had a light breakfast When overweight and obese women were put on a weight-loss diet for three months,and ate most of their calories at dinner – even though they consumed the same number of calories overall.

Early starts followed by late weekend lie-ins can scramble our body clocks (Credit: Alamy) Early starts followed by late weekend lie-ins can scramble our body clocks (Credit: Alamy) Many people think that the reason you gain more weight if you eat late at night is because you have less opportunity to burn off those calories, but this is simplistic. “People sometimes assume that our bodies shut down when asleep, but that’s not true,” says Jonathan Johnston at the University of Surrey, who studies how our body clocks interact with food.

So, what else could be going on?

Some preliminary evidence suggests that more energy is used to process a meal when it’s eaten in the morning, compared with later in the day, so you burn slightly more calories if you eat earlier. However, it’s still unclear how much of a difference this would make to overall body weight.

Another possibility is that late-night eating extends the overall window during which food is consumed.

This gives our digestive systems less time to recuperate and reduces the opportunity for our bodies to burn fat – because fat-burning only occurs when our organs realise that no more food is coming their way.

Prior to the invention of electric light, humans woke at roughly around dawn and went to bed several hours after the sun set, with almost all food being consumed during daylight hours.

“Unless we have access to light, we struggle to stay awake and eat at the wrong time,” says Satchin Panda, a circadian biologist at the Salk Institute in La Jolla, California, and author of The Circadian Code.

His own research has revealed that the majority of North Americans eat over the course of 15 or more hours each day, with more than a third of the day’s calories consumed after 6 pm, which is very different to how our ancestors must have lived.

Now consider those college students, eating and drinking long into the night.

“A typical college student rarely goes to sleep before midnight, and they also tend to eat until midnight,” Panda says. Yet, many students will still need to get up for classes the next day, which – assuming they eat breakfast – reduces the length of their night-time fast still further.

It also means that they are cutting short their sleep, and this too could make them more likely to gain weight.

Inadequate sleep impairs decision-making and self-control, potentially leading to poor food choices, and it disrupts levels of the “hunger hormones”, leptin and ghrelin, boosting appetite.