Our body clocks are central to a complex web of biological processes. So, when should we eat for optimum health? Does it matter if we eat irregularly, skip breakfast, eat late dinners, or snack? The answer, writes Hatty Willmoth, is more nuanced than we might imagine.

Clocks exist inside you; it sounds spooky, but it’s sort of true. Your body can feel the passing of time, not just instinctively, but because it’s keeping track of it.

The brain receives cues from cycles of light and dark. Whether it’s the setting and rising of the sun, a lamp switching on and off, or light blaring out from a phone screen, it all influences a central clock.

But there are also a multitude of peripheral clocks in our muscles, organs, tissues – in fact, most cells in our bodies – which calibrate not with light, but with food.

Together, all these clocks allow daily 24-hour cycles called circadian rhythms to govern how we function. Nutritionally, they influence when we eat, what we eat, and how we process food.

Nutritional therapist and ION lecturer Catherine Jeans explains: “Chronobiology is the study of biological or circadian rhythms: cyclical patterns that we see all around nature, including in humans.

“For example, one of the simplest [patterns] that humans display is the natural tendency to sleep when it’s dark and be awake when it’s light.

"Other examples are our appetite signals, digestion, immune system and body temperature."

Falling out of step with your circadian rhythm

There are many mysteries yet to be solved in chronobiology, but we do know that these internal clocks can become out of sync – with each other and our environment – and this can have negative consequences for our health.

Called circadian misalignment, perhaps the most obvious example is jetlag: a fogginess that descends when our bodies want to sleep, but the sunlight tells us it’s daytime.

Other more mundane factors can have a similar effect if they disrupt natural patterns, for instance: working nightshifts, sleep disturbances, constant access to food, and even artificial lighting.

These can send confusing signals to our body clocks and lead to circadian misalignment, sometimes known as social jetlag.

Shifting towards digestive issues

In the long term, scientists have linked circadian issues with metabolic diseases, from insulin resistance and diabetes, to obesity, heart disease and gut problems.

This is largely because circadian rhythms are central to our digestive health – and no one knows this as keenly as shift workers.

Indeed, digestive issues have been associated with shiftwork ever since the 1920s when the phrase ‘busman’s stomach’ was coined to describe digestive upset common to workers on London buses.

Today, studies of circadian misalignment often focus on people who work odd hours or during the night.

The evidence is pretty unanimous that this sort of shiftwork is linked to higher incidences of digestive issues, irritable bowel syndrome (IBS) and constipation (as well as metabolic disorders such as obesity and type 2 diabetes), and that circadian rhythm disruption is a key culprit.

For example, one study from 2017 found that 61% of nursing students working irregular shifts had constipation, compared to 31% of those working regular shifts.

Jeans says: “Our circadian rhythm is hugely significant for our digestion, affecting many of the complex processes within our digestive system.

“Irregular eating patterns can be a trigger for disrupted circadian rhythms within the bowel. This is because our digestive system is naturally more inactive during the night, and at its peak of activity during the waking hours.

“Our bowel movements also follow a natural circadian rhythm, with colonic motility [movement of the gut] slowing at night (so that you can get a good night’s sleep), and more active during the day.”

So, our guts are naturally more active during the day, but circadian rhythms also influence the production of stomach acid, absorption of nutrients, processing of sugars, and more.

Even the composition of our gut bacteria changes over the course of 24 hours.

These processes can be affected if our routines get derailed, especially if we are awake and eating at night, when our digestive systems are likely to be sluggish and inefficient.

Jetlag can have a similar effect; in a small study on 70 jetsetters from Europe to the US, researchers found that nearly 40% experienced traveller’s constipation.

It seems humans are just better at digesting food effectively when we're in tune with our circadian rhythms.

Eating in the dark

What if we don’t work nightshifts, but just enjoy a late-night snack (as 35% of us do, according to researchers at the health science company ZOE) or eat dinner long after the sun has gone down? Research suggests that even this type of nighttime eating may have detrimental effects on our health.

It’s a similar story to working a nightshift; we’re eating while our digestive systems are in sleep mode, and this may result in slower digestion, less effective absorption of nutrients, and poorer blood sugar control.

Some studies have even suggested that nighttime eating is associated with an elevated risk of breast and prostate cancers in the long term – perhaps by as much as 20%.

However, Jeans says nighttime eating may not be as bad as it seems: “The reality is, it really isn’t that simple."

Is eating late bad for you?

She says it’s true that eating at night is not optimal. “It may suit our natural circadian rhythms if we eat larger meals earlier in the day, when our digestion is more active.

“Also, if our natural circadian rhythms become disrupted, then this can affect our metabolism and increase our risk of putting on weight and developing metabolic diseases.”

However, she adds, “we have to work with where we are at” and “it might not be possible to have our evening meal before the sun goes down”.

“We need to get in enough good nutrition, and we may have more time to do that with our evening meal.”

Also, late eating may affect some more than others. Jeans explains: “As humans, we are all biologically unique, and different ‘chronotypes’ have been found when it comes to circadian rhythms, which can influence our behaviour around when we like to eat or sleep.

"There are also genetic variants in clock genes that can affect both our metabolic health and how we respond to different foods and eating patterns."

What's a chronotype?

A ‘chronotype’ is a fancy way to describe when someone likes to get up and tends to go to sleep, just like describing yourself as ‘an early bird’ or ‘a night owl’.

Most people fall somewhere in the middle, but others may feel they have a natural preference towards the morning or evening.

There’s a genetic component to it, but studies show that we can alter our chronotypes with behaviour, so it’s not set in stone.

And we may wish to change our chronotypes, because night owls get a bad rap. Having a later chronotype has been associated with irregular eating, poorer diet quality, higher rates of mental health issues (especially depression), and a greater risk of physical health issues, from obesity to type 2 diabetes and heart disease.

Studies suggest that night owls are more likely to smoke, live alone, drink greater quantities of alcohol, have erratic sleep patterns, and exercise less. In fact, they’re more likely to live shorter lives.

However, it’s unclear to what extent this is a cause-and-effect situation, and to what extent it’s societal.

On the one hand, it makes sense for our bodies to function optimally when our circadian rhythms align with those of the sun. Research shows, for example, that depression is more common among those who spend less time in natural daylight.

Yet, on the other hand, later chronotypes may be disadvantaged by a society which tends to operate to the beat of an earlier drum.

Night owls may have to keep to a schedule at odds with their preferred circadian rhythm. Studies show that many lose out on sleep during the week, only to catch up on their ‘sleep debt’ at the weekend.

But a study of factory workers has found that matching workers’ shifts to their chronotypes improved overall morale and general wellbeing, so if you can adjust your working pattern to suit your own chronotype, it may prove beneficial.

Breakfast like a king - or don't

How should our breakfasts support our circadian rhythms? Again, the answer is personal.

We’ve all heard that breakfast is the most important meal of the day, and there is evidence to suggest that eating a bigger breakfast can have health benefits.

After all, it makes sense to consume more food when the digestive system is primed to be active. But a large, early breakfast might not suit everybody – and Jeans says it shouldn’t have to.

“We don’t all need to eat a big breakfast,” she says. “We are all biochemically unique, and some people do really well with a larger breakfast when they wake up, but some people really find they cannot eat much breakfast as their digestive systems just don’t feel ready for it.

“However, it is generally recommended that we properly nourish ourselves when we break fast, whatever time you choose to make this happen.

“In my experience as a practitioner, it’s important for people to listen to their bodies. Rather than focus on the size of the breakfast, I recommend focusing on the quality of that breakfast.”

Intermittent fasting: a delayed breakfast

Some eat a late breakfast or skip the meal entirely. Paired with a late dinner, this may lead to circadian misalignment and metabolic issues.

However, if we eat dinner earlier and also delay or skip breakfast, it gives the body longer to rest and repair without food.

In other words, it extends our overnight fasting window. A form of intermittent fasting, this eating pattern has been associated with improved metabolic outcomes.

Jeans says: “There is a lot of evidence to support the use of time restricted feeding (TRF) to aid metabolic health.

“As humans, we are naturally tuned in with nature, and our digestive systems do need to rest so that they have time to repair. Therefore, we naturally could all benefit from at least a 12 hour overnight fast.

“This is relatively safe for most people to do (although it’s always important to speak to your GP if you have any pre-existing medical conditions before you make any changes to your diet).

“Some people also like to have a longer overnight fast, because more and more research is demonstrating the positive impact on many aspects of our health, including our metabolism, glycaemic control, digestive and cardiovascular health.

“However, once again, this really depends on you as a unique individual. For example, if you are experiencing high levels of stress or you are in a period of hormonal shift (such as in the perimenopause), you may not benefit from a longer overnight fast. Children also are not advised to extend their fasting periods.”

Read this article for a more in-depth understanding of intermittent fasting and all its purported benefits.

Three meals a day and that's it

Having a distinct period of not-eating in every 24-hour cycle helps us maintain healthy circadian rhythms, but what about our eating habits throughout the day?

Nutritional advice on this has taken a sharp U-turn over the past 30 years or so. Eating 'little and often’ was once a typical recommendation; however, current wisdom prefers eating three filling meals, spaced four or five hours apart without snacks.

Jeans explains why. “When we eat, our insulin levels increase to help us balance our blood sugar and get it to where it needs to be.

“If we are constantly snacking, this can lead to continually higher levels of insulin, which over time can have a negative impact on our metabolic health.”

However, research connected with the ZOE Project (a large nutritional science study and personalised nutrition programme)  has muddied those waters, finding that how often we eat has little impact on our health markers.

ZOE researchers say that what matters is not whether we snack, but what we’re snacking on – and when.

As previously mentioned, many of us reach for a little nibble late at night; this shortens our overnight fasting window, and makes our bodies start digesting when they’d rather rest.

But also, the foods we usually eat as snacks in the UK tend to be nutritionally poor, especially compared to what we eat at mealtimes.

Typical snacks – biscuits, crisps, snack bars, etc. – are often high in sugar and unhealthy fats, low in protein and fibre, and ultra-processed, so they usually don’t contribute to good health.

Indeed, the ZOE scientists estimate that a quarter of us undo the benefits of mealtime healthy eating by reaching for unhealthy snacks.

But the ZOE team also found that their study participants were more likely to be metabolically healthy if they ate healthy snacks than if they didn’t snack at all.

Plus, a trial by top ZOE scientist Dr Sarah Berry at King’s College London found that snacking on almonds could improve health. The issue is most of us don’t snack on almonds.

Jeans says: “If you do need to snack, choose slow releasing foods that contain fibre, protein and healthy fats – such as a piece of fruit and small handful of nuts, vegetable sticks and hummus.”

Is there an optimum pattern of eating?

It turns out that ‘when we eat’ is really quite important – so is there an optimum pattern of eating?

Jeans says: “As a nutritional therapy practitioner, I encourage my clients to try to tune into their own bodies.

“However, if there were an optimal pattern of eating, it would be to have at least a 12 hour overnight fast, possibly longer, and to eat with the pattern of the sun: more calories in the morning and early afternoon, less in the evening.

“Having said that, modern day humans do not all fit that pattern, and we also have to weigh this up with the benefits of sitting down as a family to eat a meal together, and how this social aspect can support our health.

“If we are not eating a proper evening meal or eating earlier, we may miss this valuable connection time.”

In other words, when we eat is personal. There are eating patterns that may be better for us biologically; but there are also eating patterns that may work better socially, and both are valid.

However, we may benefit from bearing in mind the little clocks inside us.

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