Oct 23, 2020 / by Jon Bellis / No Comments

Is there really a link between your body clock and weight gain? Most definitely. And it’s quite likely that you’re impacted. Read on to find out how.

But first, here’s a familiar perspective on your body clock.

The clocks go back in the UK this weekend. You’ll have heard people say, ‘you get an extra hour in bed!’. But will you? The chances are your body will wake you up an hour earlier, and you’ll get the same amount of sleep as usual.

Why is that? Why can’t your body adjust immediately to the new time? In the case of a one-hour clock change, you’ll hardly notice it and your body will adapt within a day or two. But you’ll feel it more if you travel across the globe and the time changes by several hours. It can take a week to get used to the new time.

Why is time zone travel so impactful?

International clocks. Your body clock and weight gain.

It’s your body clock, of course! But is your ‘body clock’ a real thing? Is there any science behind it? You bet there is! In this post, we uncover the link between your body clock and weight gain. We describe what your body clock is and explain why, when you disrupt it, it can lead to weight gain.


How your body clock works


It’s clocks, plural!

Big pile of different clocks. Your body clock and weight gain.

It makes sense that we should have a body clock. After all, there has been a daily cycle of light and dark since the earth was formed. Plants function in a cyclical way, harvesting and utilising energy according to the daily rising and setting of the sun. Unicellular creatures and fungi also have a distinct rhythm to their metabolism, following a 24-hour cycle.

Along with all mammals, you also have a clock. You express the mammalian clock gene within the suprachiasmatic nucleus (SCN) in your brain and almost all cells in the body. The SCN is the master clock, responsive only to signals of light and dark. The organs in the rest of the body have secondary clocks which are responsive both to the master clock and to other signals. These organs are outside – or peripheral – to the SCN or master clock, so we refer to them as peripheral organs. The secondary clocks influence the peripheral organs involved in energy metabolism such as your liver, pancreas, muscles and fat tissue.


Physiological rhythms

If you synchronised your secondary clocks with the master clock, you would be wakeful during the day, asleep at night, and your master clock, secondary clocks and metabolic processes would all be aligned.

Here’s a summary of what your body should be doing when the master clock and secondary clocks are in sync.

Table of physiological processes in someone whose body clocks are in sync

How it works is that the master clock exerts its influence primarily through two hormones, cortisol and melatonin, and the Central Nervous System (CNS). The secondary clocks, and therefore the peripheral organs, are influenced by these hormones and the CNS. These, in turn, regulate energy metabolism by controlling the rhythm of energy intake and activity.

They do this by altering appetite and alertness throughout the day, for example. Through this regulation of food intake, activity and metabolism, the master clock and peripheral organs orchestrate synchronised rhythms of behaviour and physiology. This, in turn, contributes to long term weight maintenance by maintaining stability.


Clock problems

That’s ideally how your body will function: in sync with the daily rhythm of day and night, light and dark. But, of course, we are autonomous. We can choose when to eat and be active; we don’t have to respond like a robot to the rhythm of the clock signals driven by light and dark. And we have busy lives: we have to do what we need to do at the time we can do it.

But that’s where the problems start. By feeding or being active at times when the body is expected to be fasting or restful, we uncouple the secondary clocks from the master clock. The rhythm of our physiology no longer matches the rhythm of our master clock, and the harvesting and utilisation of energy is at odds with the rhythm of our sleep-wake cycles.


Clock disturbances and weight gain

There is a well-established link between disrupted body clocks and obesity, and many research papers on the subject.

Although the exact mechanisms are still being figured out, you can understand why clock disturbances might lead to weight gain. You’ll disrupt energy usage and storage if your body is active when it’s receiving signals to be restful. Likewise, if it’s feeding and storing energy when it’s supposed to be using up fat stores or if it’s feeling sleepy when it’s meant to be alert, then energy balance will be impacted.

In the modern world, the number of people whose secondary clocks and energy metabolism are aligned with the central clock and day-night cycles is low. This population-wide clock misalignment is likely a contributing factor to the obesity epidemic.

So, could it be that some of your weight gain is the result of clock disruptions? Quite possibly. It’s been shown that a disrupted clock leads to increased food intake and that it’s caused by hormonal changes that lead to an increased appetite rather than extended opportunities to eat. It might be that you’ll have an easier time losing weight if you get your body clocks in order.


Other clock-related health issues

But that’s not the only consequence of a disrupted body clock. There is an association with

  • Insulin resistance and diabetes.
  • Dyslipidaemia and fat accumulation – especially around the organs.
  • Increased morbidity and mortality.
  • Higher fat intake, poor food choices and irregular eating.

So, a disrupted body clock is far more impactful than simply feeling tired or out of sorts. It could be affecting your health significantly.


You might be affected

Now, before you decide to stop reading because a disrupted body clock does not apply to you, think again. There are many ways in which your body clocks can become misaligned. Jet lag might be the mechanism that springs to mind, but it might be the least of your worries when it comes to your body clock.


Ways in which your body clock can be disrupted

The next obvious question, then, is how might your body clock be disrupted? What is causing it? And what can you do about it?

Let’s look at some common ways in which your clocks can get out of sync.


Shift work

If you work at night, you’ve really flipped your body clock on its head. You’re active and eating when it’s dark and trying to sleep when it’s light. Your master clock is influencing your body to do one thing, and your organs are being persuaded to do something different.

Unsurprisingly, there’s a clear link between shift working and metabolic health. Shift workers have been shown to have an increased risk of diabetes, obesity and cardiovascular ‘events’. Collectively they have higher insulin resistance, inverted cortisol rhythms and higher blood pressure. There are even links to an increased risk of cancer.

These studies make some interesting points:

  • There is little evidence that shift workers adapt and flip their master clock to align with their activity and feeding patterns.
  • There is an increased risk of abdominal obesity, meaning shift workers will tend to store fat around their internal organs.
  • Energy expenditure during sleep is reduced, and the heating up of the body that results from food intake (TEF – thermic effect of food) is also reduced. So, you can see how overall energy expenditure would be reduced.
  • Appetite is increased and shift workers tend to eat a higher fat diet and more unhealthy snacks.


Night at light

Do you leave a light on at night? Please don’t do it! Artificial light influences your body clock. You might be asleep, but your master clock will think it’s daytime!

It’s been shown that light at night causes weight gain. And the relationship has been well studied. It’s been suggested that night time suppression of melatonin might directly provoke metabolic dysfunction and fat accumulation.


Night eating

Night time food delivery.

Do you get up and raid the fridge in the middle of the night like Nigella Lawson? Perhaps not. But you don’t need even to do that to disrupt your body clock. If you eat a big meal quite late, just before bed, you’re sending the wrong signals. It’s far past sunset, it’s dark, and your body clock is telling you to rest and fast. Instead, you’re feeding.

So, eating during your biological night, or consuming most of your daily calories in the evening may contribute to fat storage. Not surprisingly, again, there is an association between night eating and obesity.

Night eating also covers night drinking, of course. It’s well known that alcohol disrupts your sleep. It acts by directly altering the master clock.


Social jetlag and sleep debt

Social jetlag is a term used to describe shortened sleep caused by lifestyle, with a subsequent catch up at weekends. This is a typical pattern these days. You may work long hours during the week, get to bed late, wake up early and do this five days a week. Then at the weekend you have a lie-in and ‘catch up’. You do this for most of the year. Although there is some evidence that the catch up is beneficial, you’re constantly providing your body clock with mixed signals, so it’s in a constant state of disruption. As you might expect, this behaviour is associated with obesity.

But it’s not just a lack of sleep through social jetlag that can cause your body clock to get out of sync. Sleep debt, in general, will have the same effect. If you are a poor sleeper or a short sleeper, you may experience the same disruption of your body clock and suffer from the same consequences. Short sleepers attempting to trim down lose less fat and more muscle and have a greater risk of diabetes, metabolic syndrome and obesity. To learn all about sleep and how you can improve it, take a look at our previous blog on sleep.


Electronic devices

Tired person working late on electronic device.

You’ve probably heard about this one. Devices such as laptops, gaming consoles and tablets emit light at just the right wavelength to blunt the onset of melatonin, disrupt your body clocks and so disrupt sleep. Your body is fooled into thinking its daytime, when in fact you should be winding down and getting ready to sleep. The resulting poor sleep, as we’ve seen, can make you at higher risk of metabolic disease and obesity.


Agitating your chronotype

Some people’s clocks run a little later than others. They may wake later and go to bed later. You might have heard those people referred to as ‘owls’. In contrast, those who go to bed early and get up early are referred to as ‘larks’. What’s your chronotype – are you a lark or an owl?

Teenagers tend to be owls, and older people tend to be larks. That’s just the consequence of the way their body clock synchronises their physiology to the signals of light and dark. But if you try asking a teenager to go to bed early and get up early, you’ll see a distinct change in behaviour! Similarly, older people will struggle to stay up late and lie in. I can vouch for this! I might ‘only’ be 53, but my eyes are drooping by 10:30 pm. And it doesn’t matter how late I go to bed; my clock wakes me up early.

If you’re naturally one type of chronotype, but you’re living a lifestyle of the other, then your clocks are going to be out of sync, and you won’t be at your best. What’s more, you’re likely to have a harder time maintaining a healthy weight.

And that’s not all. If you’re an owl, it’s bad news, I’m afraid. Owls tend to be more out of sync with the rhythm of natural daylight, and there’s evidence that owls have a tougher time being metabolically healthy.



International travel, taxi-ing plane. Your body clock and weight gain.

If you fly east or west by several hours, you’ll arrive in a different time zone. Your body clock is synchronised to the time where you started your journey. I know from experience that this disruption of your master clock can be debilitating.

The good news is that light and dark are synchronised to your new time zone, so your master clock can adapt to the new time within a few days. That means international travel once or twice a year is not going to cause too much of a long term impact.

But, if you travel a lot, say once or twice a month, then your body clock is going to be disrupted more often than it is going to be in harmony. Remember that, if it takes a week to adjust on the outward journey, it might also take a few days to adjust back when you return. That’s around ten days of disruption. Do that twice a month, and you’re more disrupted than synchronised.


Seasonal changes

If you live a long away from the equator, the length of night and day will change throughout the year. As the seasons change, humans experience changes in activity, energy intake and meal nutrient composition. These are thought to be residual evolutionary influences of hibernation.

Historically, longer summer days allowed more time to feed and store energy for winter. Winter days were characterised by low levels of activity, less energy harvesting and using up of energy stores.

Now we have artificial light, and our days are the same length all year round. We tend to be less active in the winter but maintain the usual intake of energy. We may even favour high-fat foods and increase energy intake. The evidence bears out this behaviour – we tend to have greater fat accumulation in winter.


What can you do about it?

As you’ve seen, there are many ways in which your secondary physiological body clock can become out of sync with your master body clock, and there’s plenty of evidence that this can cause weight gain and health complications.

So, what can you do to prevent or negate the effects of a disrupted body clock? Here are some ideas.


Change your job!

If you work at night, find a job that is better aligned to the light-dark cycle.


Keep your bedroom dark

Turn off any night lights, don’t put the light on when you go for a pee in the night, and get some blackout blinds.


Avoid night eating

Definitely don’t get up in the night to eat. But also try not to eat too close to bedtime, or at least have a smaller meal at night.


Avoid alcohol close to bed

Alcohol, in general, will disrupt your physiology as well as your body clock. It will cause a rise in cortisol and change the way your energy metabolism functions. Ideally, don’t drink at all if you want your body clock to remain stable. If you do drink, try not to drink too close to bedtime.


Breakfast like a king, lunch like a prince, dinner like a pauper

Your body is awake and alert at breakfast, and it should be sleepy and restful later in the day. Match your food intake to your body clock’s rhythm.


Read a book before bed

Switch off your laptop and game console. Read a book instead.


Respect your chronotype

If you’re a natural owl, then behave like one. If you’re a natural lark, then be a lark.


Even out your sleep

If you can, get more sleep during the week and avoid that weekend lie-in. In short, be consistent with your sleep-wake cycles.


Exercise at the right time

Try not to exercise too close the bedtime. The evening should be wind-down time. If you have to exercise after work, try and make it the late afternoon or early evening. Exercising can maintain or reset your master clock. I used to use this strategy when I travelled abroad. If I woke up early due to jetlag, I’d get up and work out at silly o’clock in the morning, then stay awake. That used to reset my body clock much quicker than if I worked out after work.



If you feel your body clock is disrupted, you could try a lightbox. Use it earlier in the day to entrain your body clock to better align with the natural light-dark cycle of the day.



Again, if your body clock is unsettled, you could try taking melatonin in the late afternoon or early evening to advance the onset of sleep. It goes without saying that you should consult with your doctor before trying this, especially if you are on medication.


Travel in the dark

If you’re a night worker, try not to get any sun exposure on the commute home before bed. Also ensure your bedroom is dark.


Notice your seasonal differences

Become aware of differences in your activity levels and eating habits in the winter compared to the summer. Try and stay just as active in the winter and limit high-calorie comfort foods.


Keep your bedroom cool

Like nutrient intake and activity, temperature can also influence your secondary clocks. Keep your bedroom cool to ensure you get a restful sleep.


Move to the equator

Only kidding! Who doesn’t love the changing seasons anyway?



You may think that your body clock is not causing you any problems. But there are many ways that your master body clock can be out of sync with either the natural light-dark cycle or your eating and activity schedule.

Many studies confirm an association between obesity and body clock disruption in its many forms. If you’re struggling with your weight and you think you’re doing all the right things, then consider whether your body clock is in good order.

The good news is that there are lots of things you can do to rectify a disturbed body clock.

But even if you recognise that your body clock is disrupted, and there’s nothing you can do about it, there is a tried and tested way to overcome the potential consequence of weight gain. Despite how your body clock might change your metabolism and behaviours, your weight is ultimately governed by your intake and utilisation of energy. Good old exercise and healthy eating will ensure you stay in good health and avoid weight gain.

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