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 15lbs 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: 7-9am was the time of the stomach, when the biggest meal of the day should be consumed; 9-11am centred on the pancreas and spleen; 11am-1pm 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.
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Although the explanation is different, modern science suggests that there is plenty of truth in that ancient wisdom.
Dieters consumed most of their calories at breakfast lost two and a half times more weight than those who had a light breakfast and ate most of their calories at dinner
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? When overweight and obese women were put on a weight-loss diet for three months, those who consumed most of their calories at breakfast lost two and a half times more weight than those who had a light breakfast 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)
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.
The majority of North Americans eat over the course of 15 or more hours each day
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 6pm, 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.
Both the number of calories you consume - and when you consume them - might be important factors for weight gain (Credit: Getty)
It is now becoming clear that our circadian rhythms are intimately connected to our digestion and metabolism in many other ways, through the body’s intricate signalling pathways – a new understanding that could explain the long-term effects of jet lag and shift work.
Inside every cell of your body, there ticks a molecular clock that regulates the timing of pretty much every physiological process and behaviour, from the release of hormones and neurotransmitters, to your blood pressure, the activity of your immune cells, and when you feel more sleepy, alert, or depressed. These clocks are kept in synchrony with each other, and with the time of day outside, through signals from a small patch of brain tissue called the suprachiasmatic nucleus (SCN). And its window on the outside world are a subset of light-responsive cells at the back of the eye called intrinsically photoreceptive retinal ganglion cells (ipRGs).
The point of all these “circadian” clocks is to anticipate and prepare for regular events in our environment, such as the arrival of food. It means that different biochemical reactions are favoured at various times of day, allowing our internal organs to task-switch and recuperate.
When we travel abroad, the timing of our light exposure changes, and our body clocks are pulled in the same direction – although the clocks in different organs and tissues adapt at different rates. The result is jet lag, which not only leaves us feeling sleepy or awake at the wrong times, but can also trigger digestive problems and general malaise.
However, light isn’t the only thing that can change the timing of our clocks. When we eat our meals can also shift the hands of the clocks in the liver and digestive organs, even though the clocks in our brain cells are unaffected. Recent evidence also suggests that the timing of exercise can tweak the clocks in our muscle cells.
International travel disrupts our circadian rhythms and the timings of our meals - and if that occurs regularly it could have long-term effects on your health (Credit: Getty)
When we fly across time zones, or eat, sleep and exercise at irregular times, the various clocks in our organs and tissues fall out of synchrony with one another. This is unlikely to be a problem if you just have the occasional late-night meal or lie-in, but if it’s a regular occurrence this may have longer-term consequences for our health.
Complex processes, such as the metabolism of fats or carbohydrates from the diet, require the coordination of numerous processes occurring in the gut, liver, pancreas, muscle and fatty tissue. If the conversation between these tissues becomes scrambled, they become less efficient, which over the long term may increase our risk of various diseases.
In one recent study, researchers compared the physical effects of sleeping for five hours per night for eight days in a row, with getting the same amount of sleep but at irregular times. In both groups, people’s sensitivity to the hormone insulin dropped and systemic inflammation increased, escalating the risk of developing type 2 diabetes and heart disease. However, these effects were even greater in those who were sleeping at irregular times (and whose circadian rhythms were therefore knocked out of alignment): in men, the reduction in insulin sensitivity and increase in inflammation doubled.
That could be a problem for frequent flyers, students who regularly sleep in, or any shift workers. According to European and North American surveys, some 15 to 30% of the working population is engaged in some form of shift work, which often equates to eating or being active when the body isn’t expecting it. Shift work has been linked to a host of conditions, including heart disease, type 2 diabetes, obesity and depression, and circadian disruption brought about by this irregularity is a prime suspect.
However, we are all shift workers at least some of the time, says Panda. An estimated 87% of the general population maintains a different sleep schedule on weekdays, compared to weekends, resulting in social jet lag. People also tend to eat breakfast at least an hour later at the weekends, which can result in so-called “metabolic-jetlag”.
You may benefit from consuming most of your calories earlier in the day, when your metabolism is at its most efficient (Credit: Getty)
It’s not only consistency in the timing of meals, but in the amount of food we eat at each meal that seems to be important.
Gerda Pot is a nutrition researcher at King’s College London, investigating how day-to-day irregularity in people’s energy intake affects their long-term health. She was inspired by her grandmother, Hammy Timmerman, who was rigorous about routine. Each day she’d eat breakfast at 7am; lunch at 12.30pm, and dinner at 6pm. Even the timing of her snacks was intransigent: coffee at 11.30am; tea at 3pm. When Pot came to visit, she soon learned that sleeping in was a mistake: “If I woke up at 10am, she’d still insist I ate breakfast, and then we’d be having coffee and a cookie half an hour later,” she says. Increasingly, though, she is convinced that her grandmother’s rigid routine helped keep her in good health until she was almost 95.
There are some good reasons why this might be. Our sensitivity to the hormone insulin, which enables the glucose from the food we eat to enter our cells and be used as fuel, is greater during the morning than at night. When we eat late (as Hammy Timmerman never did), that glucose remains in our blood for longer, which over the long term can increase the risk of developing type 2 diabetes, where the pancreas no longer produces enough insulin. It can also damage tissues elsewhere, such as blood vessels or nerves in the eyes and feet. In the worst cases, this can result in blindness, or amputations.
Eating at irregular times can contribute to conditions such as diabetes (Credit: Getty)
Using data from a UK national survey which has tracked the health of more than 5,000 people for over 70 years, Pot found that, even though they consumed fewer calories overall, people who had a more irregular meal routine had a higher risk of developing metabolic syndrome – a cluster of conditions, including high blood pressure, elevated blood sugar levels, excess fat around the waist and abnormal fat and cholesterol levels in their blood, which together increase the risk of cardiovascular disease and type 2 diabetes.
So, what should we do about it? Striving for greater consistency in the timing of our sleep and meals is a good first step, and ideally, all our clocks should be operating on the same time zone. When we open the curtains and see bright light in the morning, this resets the master clock in the brain, so by eating breakfast soon afterward, this reinforces the message that its morning to the clocks in our liver and digestive system. Eating a good breakfast may therefore be essential to keeping our circadian clocks running in synchrony.
Indeed, a recent study involving 18 healthy individuals, and 18 with type 2 diabetes, found that skipping breakfast led to disrupted circadian rhythms in both groups, as well as greater spikes in blood glucose levels when they finally did eat.
However, regularising our schedules shouldn’t come at the expense of missed sleep. Although it’s unlikely that the occasional lie-in will cause you any harm, we should generally be striving to go to bed at a time that will allow us to get adequate sleep – the recommended amount is seven to eight hours for most adults – on every day of the week. Here, light exposure could help. Dimming the lights in the evenings and getting more exposure to bright light during the day time has been shown to shift the timing of the master clock in the brain (the SCN) several hours earlier, making people more lark-like. (Read more: What I learnt by living without artificial light.)
The body's metabolism slows throughout the day, meaning that late-night takeaways may be especially bad for dieters (Credit: Alamy)
Some are advocating a more hardcore approach of forgoing all food for at least 12 hours, and possibly for as long as 14-16 hours overnight. In a landmark study published in 2012, Panda and his colleagues compared one set of mice that had access to fatty and sugary foods at any time of day or night, with another group that could only consumed these foods within an eight to 12-hour window during their “daytime”. Even though they consumed the same number of calories, the mice whose eating window was restricted appeared to be completely protected from the diseases that began to afflict the other group: obesity, diabetes, heart disease and liver damage. What’s more, when mice with these illnesses were placed on a time-restricted eating schedule, they became well again.
“Almost every animal, including us, evolved on this planet with a very strong 24-hour rhythm in light and darkness, and the associated rhythms in eating and fasting,” explains Panda. “We think a major function [of these cycles] is to enable repair and rejuvenation each night. You cannot repair a highway when the traffic is still moving.”
Human trials of time-restricted eating are just beginning, but some of the early results look promising – at least in certain groups. For instance, when eight men with prediabetes were randomised to eat all their meals between 8am and 3pm, their sensitivity to insulin improved and their blood pressure dropped by 10-11 points on average, compared to when they consumed the same meals within a 12-hour period.
Quite what this means for the rest of us is unclear at this point, but the adage that you should breakfast like a king, lunch like a prince and dine like a pauper has never seemed truer. And it’s almost certainly worth fitting a padlock on the fridge overnight.
Linda Geddes is a science journalist and the author of Chasing The Sun: The new science of sunlight and how it shapes our bodies and minds, published by Wellcome Collection.
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