It is close to midnight and I am bingeing both on a Netflix show and leftovers from the fridge. I know I will regret it in the morning – and perhaps for years to come.
With my late-night feasting, I am inadvertently playing fast and loose with a system that evolved to keep my body in sync with the natural 24-hour cycle of day and night. Its effects include setting optimum times to eat, and there is emerging evidence that overriding this schedule can have severe consequences for health.
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The syncing begins in a small cluster of neurons in the brain called the suprachiasmatic nucleus (SCN), which is the body’s central timekeeper. Its function is to make sure biological processes such as falling asleep occur at the optimal time. The SCN is reset daily by light and darkness, creating a roughly 24-hour cycle called the circadian rhythm. It also coordinates appetite to ensure that we have adequate supplies of energy during the day and can get through a night’s sleep without being awoken by digestion or hunger.
This translates into natural peaks in appetite in the morning and early evening. Processes involved in the digestion, absorption and metabolism of food are similarly coordinated by the SCN to be primed for action at the appropriate times.
But the SCN doesn’t solely call the shots. The body also has numerous secondary clocks in peripheral tissues and organs that largely take their orders from the central clock, but also respond to external cues called zeitgebers (German for “time givers”). The , which causes the release of hormones involved in metabolism and satiety. Zeitgebers also reset the peripheral clocks in the gut, liver, pancreas, heart and adipose tissue.
Out of sync body clocks
Ideally, the central and peripheral clocks are in sync. But eating at the wrong time can create a mismatch. That spells trouble for our waistlines and general health. Feasting when the SCN wants us to fast has been . Part of the problem may simply be that eating out of hours means we consume too many calories, according to at the German Institute of Human Nutrition Potsdam-RehbrĂĽcke.
But timing also appears to be a crucial factor: a of night-shift workers found that they were more likely than day-shift staff to become overweight or obese even when consuming the same number of calories.
Overriding the body's natural 24-hour cycle can have severe health consequences
Despite this clear association between meal timing and health, “the underlying mechanisms remain largely unknown”, according to Pivovarova-Ramich. We know that thermogenesis – the generation of body heat through metabolism – is under circadian control and is higher in the morning. This is also true of our resting metabolic rate, the amount of energy our bodies use in the absence of exercise. It could be that, because of these factors, calories consumed in the “biological night” are shunted into fat storage rather than burned.
In today’s world, there are endless opportunities to grab a tasty zeitgeber at the wrong time. Many of us live in 24-hour societies where food is widely available at all hours. Artificial light, such as from TV screens, may also play a role. It can throw the SCN out of whack, adding another source of confusion between the central and peripheral clocks. Artificial light can also extend the active portion of the day and shrink the biological night, which studies in mice suggest may .
So, when is the right and wrong time to eat? It depends to some extent on individual biology. Everyone has a “chronotype“, which is their largely innate preference for when their waking hours start and end. Around 20 per cent of us are “owls”, who naturally wake late, go to bed late and function better in the afternoon. Another 20 per cent are “larks”, who wake and sleep early. The rest lie somewhere in the middle.
These different chronotypes are largely a result of genetic differences that affect circadian rhythms. But as a general rule of thumb, it is wise to get most of your calories earlier in the day and avoid eating altogether during your biological night, which for non-owls starts around 8pm. have shown that night eaters, who consume the majority of their calories after this time, tend to have a higher body mass index.
Time-restricted eating
One way to avoid eating too late is through time-restricted eating (TRE), a popular form of fasting where all the day’s calories are consumed within a limited and consistent window, typically 8 to 10 hours. In practice, that means either fasting throughout the morning or stopping eating in the late afternoon, which are known as late and early TRE respectively. Many studies show that both versions have ranging from better blood sugar control to healthier cholesterol levels.
Exactly why isn’t clear. “Unfortunately, there’s not been much mechanistic work on time-restricted eating in humans,” says at the University of Alabama at Birmingham. But circadian rhythms appear to be involved. TRE has been shown to extend lifespan in animals, though some research has cast doubt on the practice, with links to increased risk of death from heart disease. What’s more, we haven’t followed people on these diets for long enough to know if TRE has longevity effects in humans, but people practising it consistently lose a bit of weight, says Peterson. There are also positive effects on blood sugar control and blood pressure, she says. Tellingly, though, these are only seen with early TRE, perhaps because it aligns better with circadian rhythms.
Another good reason to ditch the midnight feasts.
