It’s easy to hate clocks. Their unstoppable forward churn wakes us up and shames us for running late. They are a constant reminder that every enjoyable moment, just like life itself, is ephemeral. But even if we rounded up all our time-telling devices and buried them deep in the earth, we could never escape clocks. Because we are one.
We don’t need to have studied the intricacies of circadian rhythms to know that we are ravenous at certain times and not others, that the mid-afternoon slump is real, and if we party until 4am we’re unlikely to sleep for eight hours afterwards, because the body clock has no sympathy for hangovers. But to better understand this all-encompassing daily cycle is to truly know our animal selves.
Most of us are awake for 16-17 hours each day, during which we never stop changing, biologically speaking. Every minute, says Debra Skene, a professor in chronobiology at the University of Surrey, “our bodies are different”. She is referring not just to our chemical makeup, bodily functions and energy levels, but also our motivations, behaviour, mood and alertness. “At every point in time, we’ve got rhythms that are either going up or going down. Some are at their peak, some are at their middle point. It’s a dynamic system.”
Some of us are early-rising larks and others are midnight-oil-burning owls, because our intrinsic clocks are unique to us. These different chronotypes, as they are known, are normal genetic variations, says Skene. Some people run a little fast, others a little slow; left unchecked, they would slip further ahead or behind.
“Over time, you’d be really desynchronised with life on Earth,” she says, “so the role of light and dark is critical to reset your clock to 24 hours each day.” The light-dark cycle “is the strongest, most consistent signal that all animals have evolved to respond to”. This is why we’re becoming increasingly aware of the negative effects of too much artificial light at night: it confuses our systems, just as it does for migrating birds and sea-turtle hatchlings.
Your body has been preparing to wake up for an hour or two before it happens. Photograph: Posed by model; ArtistGNDphotography/Getty Images
While we’re all kept more or less in line by the same 24-hour light cycle, our different chronotypes mean some of us prefer to wake up and go to bed earlier or later than others. Skene’s team found that even if a night owl trains their body clock to comfortably run two hours earlier, by strictly adhering to regular waking, bed, breakfast and lunch times, when they stop that training, “they might drift back to being late types”, in accordance with their internal clocks.
The circadian clock is designed to ensure survival. To do this, it needs to anticipate what is going to happen. It doesn’t respond to you waking up; it has been secretly preparing your body to wake up an hour or two before it happens. “Your cortisol hormone, which is directly driven by the master clock in your hypothalamus, is beginning to rise, so by the time you wake up, it is nearly at its peak,” says Skene. “And you need the cortisol because it provides a source of glucose, and gives you the courage to get up and face the world.”
Any fundamental change in your behaviour as you move through the day, says Robert Lucas, the director of the centre for biological timing at the University of Manchester, “has to involve coordination across lots of different aspects of your body. So let’s say there’s a time of day when you are likely to be hungry and looking forward to a big meal. That’s a motivational change in your brain, but you also need to have a coordinated change in your digestive system and your liver to predict that that food is going to arrive.”
It’s your biological clock that keeps track of time and makes sure everything happens when you need it. If you keep chopping and changing your routine, Lucas says: “This coordination can fall apart, and the predictive ability of your body doesn’t work very well. We can experience it to a minor extent even when the clocks change, but definitely with jet lag.” Skene’s team at Surrey found that if you eat at midnight, food won’t be metabolised the same way it would if you ate at midday, resulting in higher levels of triglycerides (fat) circulating in your blood.
There’s a whole system of alertness at play, too, and the battle to maintain it becomes harder the longer we’re awake. “Even if you’ve had a good night’s sleep,” says Skene, “you’ve got something counting the hours of being awake, like an hourglass. Your sleep pressure builds up all day.” But with such a long day, we need an extra boost to get us safely through the last part. So late afternoon, or early evening, we experience a secondary peak in energy and cognitive function. “That’s our circadian rhythm in alertness,” says Skene, helping you stay awake until bedtime.
Deer change with the seasons – but do humans? Photograph: Jared Lloyd/Getty Images
If light is regulating our master clocks, surely seasonal swings in daylight hours would change our behaviour? Skene says: “When the dawn and dusk are changing, we’ve got a bit of flexibility in the system. Animals like sheep and deer change their reproductive capacity, skin colour and body weight based on seasons. The big question we are still trying to answer is: how seasonal are humans?” It’s tricky to study, she says, because “we’ve so changed our environment, our body doesn’t know it’s dark winter out there, because we’ve got lights and heating on. So we think we’ve got the capacity to be seasonal, but the way we’ve changed our world now, it’s hard to detect.”
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The more holistic biologists’ understanding of the body becomes, the more juicy clues to the true complexity of our body clocks emerge. The gut microbiome has its own circadian rhythm, for example. Its daily routines interact with ours as it carries out tasks including helping to digest and extract nutrients after we’ve eaten, and making neurotransmitters such as serotonin. Even our mitochondria, the energy sources within our cells, have their own circadian rhythms, according to researchers at University College London (UCL).
Their 2019 paper, A Day in the Life of Mitochondria, showed that they “kick in really hard early in the mornings”, says Glen Jeffery, a professor of neuroscience at UCL. “They know dawn is coming when we’re still asleep.” That’s when they start generating energy, “so they’re getting you ready. This probably goes back to our evolutionary state – when you wake up early in the morning, you’re really vulnerable. Something could have been watching you during the night. You want to get up and you want to be very functional.”
Mitochondria, Jeffery suspects, perform many other crucial bodily tasks we have yet to pin down, but we do know they have a big say in ageing and death, so they’re pretty fundamental. The energy they produce comes in the form of ATP (adenosine triphosphate), a chemical present in your cells. ATP is constantly being produced and burned up. “You make your body weight in it every day,” says Jeffery. “It’s a vast process – you do not do anything in this world without ATP.” As ATP starts peaking in the morning, so does our metabolism. “Your metabolism is in a very fast state,” says Jeffery. “You may not feel like it when you crawl out of bed, but it is.”
Around midday, the mitochondria start slowing down, producing less energy, and by the evening they are much less active. Jeffery says this is why muscles can ache more after evening exercise. At night, when mitochondria-produced ATP is low, your body can produce ATP energy a second way; but, says Jeffery, it uses “this bad pathway, which is called glycolysis. Glycolysis is terribly inefficient, and when you go for a run late in the day, and you run really hard, and all your muscles hurt, that’s because of glycolysis. Glycolysis is like an old Ford Cortina. It moves, but it produces a lot of crap.” By crap, he means pro-inflammatory substances.
Running at night may make your muscles ache more. Photograph: Posed by model; dusanpetkovic/Getty Images
Like our circadian rhythms, mitochondrial body clocks are led by sunlight. “They watch light all the time,” says Jeffery, “and what makes them super exciting is they talk to one another. So if I start messing with mitochondria in your toe, by the following morning, mitochondria in the rest of your body know exactly what’s going on.”
Jeffery spent his early career in the Arctic, looking specifically at how animals deal with light and darkness. He noticed that when it was dark all day, his colleagues would not only turn on the lights, but they were “very fond of having fires. A fire produces the same wavelength of light as the sun.”
“Mitochondria are a battery,” he continues. “You could put an electrode across them and see the charge. When the charge runs down far enough, they signal cell death. And if there’s enough mitochondria signalling cell death, the organism dies.” Sunlight helps charge those batteries.
Earlier this year, his team published a paper demonstrating that “if I take you outside in normal sunlight, and I put a spectrometer and a radiometer in your back, and I face you to the sun, I can measure long wavelengths of light coming through your body that are improving your mitochondrial function.” Even on a cloudy day, it may feel as though there isn’t any sunlight. Not so, says Jeffery. In fact, he says: “Those long wavelengths of light that mitochondria need are scattered by the cloud, so it doesn’t matter that it’s a cloudy day. It’s not that significant.”
Lucas’s team at Manchester are investigating the importance of daytime light, and whether going outside to maximise it can help counter the confusing effects on our daily bodily rhythms caused by artificial light in the evenings.
Softer light may be beneficial in the evening.
Photograph: Posed by model; Westend61/Getty Images
“There’s an understanding that for these biological clocks it is damaging to be exposed to light in the evening and at night,” he says. “But the other thing that’s happened is, because of electric lighting, we can spend most of our days indoors, and that means we’re also not exposed to the natural, very bright daytime light that we would have been exposed to throughout our evolutionary history. Changing daytime light exposure, for most people, is going to be a more tractable thing than changing their evening and night-time light exposure, right?” In other words, it’s harder to persuade people to give up watching TV or using social media in the evening than it is to cajole them into going for a walk outside during the day.
It’s all about routine – which, says Lucas, is highly individual, making it hard to generalise about precise biological coordinates throughout the day. “As soon as you say, ‘People sleep best at night,’ there will be somebody who says, ‘Actually, I really like being awake until four.’ There are big inter-individual differences in these things.” He has noticed his own routines change as he has got older. “I’m reliably awake now at six in the morning. When I was 18, I wasn’t. So they definitely are flexible and malleable.”
But the universal truth, says Lucas, “is that everybody will experience these rhythmic changes in pretty much every aspect of their body”. And perhaps, to stay better in tune with ourselves, it pays to remember the complex, body-wide changes occurring beyond what we feel at any given moment.
“Your lived experience might be feeling sleepy,” says Lucas, “but buried underneath that are a lot of things for your body to get ready. The same goes for when you’re hungry, frisky and all of those things.”
