IIt's easy to hate watches. Their unstoppable flow forward wakes us up and shames us for being late. They are a constant reminder that every pleasant moment, like life itself, is ephemeral. But even if we collected all our time-telling devices and buried them deep in the ground, we would never be able to escape the clock. Because we are one.
We don't need to study the intricacies of circadian rhythms to know that we feel hungry at certain times, that the midday slump is real, and if we're partying 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, you need to truly know your animal self.
Most of us are awake for 16 to 17 hours every day, during which time we never stop changing from a biological perspective. Every minute, says Debra Skene, professor of chronobiology at the University of Surrey, “our bodies are changing.” She refers not only to our chemistry, bodily functions, and energy levels, but also to our motivation, behavior, mood, and alertness. “At every moment in time we have rhythms that are either rising or falling. Some are at a peak, some are at a midpoint. It is a dynamic system.”
Some of us are early risers and others are oil-burning night owls because our internal clocks are unique. These different chronotypes, as they are called, are normal genetic variations, Skeen says. Some people run a little faster, others a little slower; if left unchecked, they will slide further forward or backward.
“Over time, you really get out of sync with life on Earth,” she says, “so the role of light and dark is critical to resetting your clock by 24 hours every day.” The cycle of light and darkness “is the most powerful and consistent signal to which all animals have evolved to respond.” That's why we're increasingly aware of the negative effects of too much artificial light at night: it confuses our systems, just like it does for migratory birds and baby sea turtles.
Although we are all more or less subject to the same 24-hour light cycle, our different chronotypes mean that some of us prefer to wake up and go to bed earlier or later than others. Skeen's team found that even if an owl trains their body clock to comfortably work two hours earlier by strictly adhering to regular wake-up, bedtime, breakfast and lunch times, when they stop this training, “they may revert back to being late again” according to their body clock.
The circadian clock is designed to ensure survival. To do this, he needs to foresee what will happen. He doesn't react to your awakening; it secretly prepares your body to wake up an hour or two before it happens. “Your hormone cortisol, which is directly controlled by the master clock in the hypothalamus, starts to rise, so by the time you wake up it's almost at its peak,” says Skeen. “And you need cortisol because it is the source of glucose and gives you the courage to stand up and face the world.”
Any fundamental change in your behavior throughout the day, says Robert Lucas, director of the Center for Biological Timing at the University of Manchester, “has to involve the coordination of many different aspects of your body. Let's say there's a time of day when you're probably hungry and looking forward to a big meal. It's a motivational change in your brain, but you also need to have coordinated changes in your digestive system and liver to predict that that food will arrive.”
This is your body clock that keeps track of time and ensures that everything happens when you need it to. If you keep cutting back and changing your daily routine, Lucas says, “That coordination can get disrupted, and your body's ability to predict won't work very well. We may experience this to a small degree, even as the clocks change, but definitely with jet lag.” Skeen's team from Surrey found that if you eat at midnight, food will not be digested as well as if you ate at midday, resulting in higher levels of triglycerides (fats) circulating in your blood.
There's also a whole vigilance system involved, and the struggle to maintain it becomes increasingly difficult the longer we're awake. “Even if you get a good night's sleep,” Skene says, “you have something ticking your waking hours, like an hourglass. Sleep pressure builds throughout the day.” But with such a long day, we need an extra boost to get through the last part safely. So, in the late afternoon or early evening, we experience a secondary peak in energy and cognitive performance. “This is our circadian rhythm of alertness,” says Skin, helping you stay awake until bedtime.
If light controls our master clock, surely seasonal variations in daylight hours will change our behavior? Skeen says: “When sunrise and sunset change, we have some flexibility in the system. Animals such as sheep and deer change their reproductive rate, skin color and body weight depending on the time of year. The big question we are still trying to answer is: How seasonal are humans?” It's difficult to study, she says, because “we've changed the environment so much that our body doesn't know it's dark in winter because we have lights and heat. So we think we have the ability to be seasonal, but the way we've changed our world now is hard to detect.”
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The more holistic biologists' understanding of the body becomes, the more interesting the clues to the true complexity of our biological clocks become. For example, the gut microbiome has its own circadian rhythm. Its daily life interacts with ours as it performs tasks such as helping to digest and extract nutrients after we eat and producing neurotransmitters such as serotonin. Even our mitochondria, the energy sources in our cells, have their own circadian rhythms, according to researchers at University College London (UCL).
Their 2019 article, A day in the life of mitochondriahave shown that they “start working very hard early in the morning”, says Glen Jeffery, professor of neuroscience at UCL. “They know that dawn is coming while we are still sleeping.” That's when they start generating energy, “so they're preparing you. It probably goes back to our evolutionary condition – when you wake up early in the morning, you're really vulnerable. Something could be watching you during the night. You want to get up and you want to be very functional.”
Mitochondria, Jeffrey suspects, perform many other essential tasks in the body that we have yet to define, but we know they play a big role in aging and death, so they are pretty fundamental. The energy they produce comes in the form of ATP (adenosine triphosphate), a chemical found in your cells. ATP is constantly being produced and burned. “You add weight to your body every day,” says Jeffrey. “It’s an extensive process—you don’t do anything in this world without ATP.” ATP peaks in the morning, and so does our metabolism. “Your metabolism is in a very fast state,” says Jeffrey. “You may not feel like it when you crawl out of bed, but that’s how it is.”
Around noon, mitochondria begin to slow down, producing less energy, and by evening they are significantly less active. Jeffrey says this is why muscles can feel more sore after an evening workout. At night, when the level of ATP produced by mitochondria is low, your body can produce ATP energy in a second way; but, says Jeffrey, it uses “this bad pathway called glycolysis. Glycolysis is horribly inefficient, and when you go out for a run at the end of the day and you run really hard and all your muscles hurt, it'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.
Like our circadian rhythm, the mitochondrial body clock is controlled by sunlight. “They watch the light all the time,” says Jeffrey, “and what makes them very interesting is that they talk to each other. So if I start messing with the mitochondria in your toe, the next morning the mitochondria in the rest of your body will know exactly what's going on.”
Jeffrey began his career in the Arctic studying how animals cope with light and darkness. He noticed that when it was dark all day, his colleagues not only turned on the lights, but also “really liked to light fires. Fire emits the same wavelength of light as the sun.”
“Mitochondria are a battery,” he continues. “You can put an electrode on them and see the charge. When the charge drops far enough, they signal cell death. And if there are enough mitochondria signaling cell death, the body dies.” Sunlight helps charge these batteries.
Earlier this year, his team published a paper demonstrating that “if I take you outside in normal sunlight, put a spectrometer and a radiometer behind your back, and turn you to face the sun, I can measure the long wavelengths of light passing through your body that improve your mitochondrial function.” Even on a cloudy day, it may seem like there is no sunlight. “That's not true,” says Jeffrey. What he's really saying is, “Those long wavelengths of light that the mitochondria need are scattered by the cloud, so it doesn't matter that it's a cloudy day. It's not that important.”
Lucas' team in Manchester is researching the importance of daylight and whether getting outside to make the most of it can help counter the disorienting effect on our daily bodily rhythms caused by artificial light in the evenings.
Photograph: Supplied by model; Westend61/Getty Images
“There is an understanding that exposure to light in the evening and at night is detrimental to this body clock,” he says. “But what has also happened is that because of electric lighting, we can spend most of our days indoors, which means that we also No exposure to natural, very bright daylight to which we have been exposed throughout our evolutionary history. Changing the light during the day will be an easier task for most people than changing the light in the evening and at night, right?” In other words, it is harder to persuade people to stop watching TV or using social media in the evening than to persuade them to go for a walk outside during the day.
It's all about routine, which Lucas says is very individual, so it's difficult to generalize about the exact biological coordinates throughout the day. “As soon as you say, 'People sleep best at night,' there will be someone who says, 'Actually, I really like staying up until four.' There are large inter-individual differences in these things.” He noticed that as he got older, his daily routine changed. “Now I wake up at six in the morning. When I was 18, that wasn't the case. So they're definitely flexible and pliable.”
But the universal truth, Lucas says, “is that everyone will experience these rhythmic changes in virtually every aspect of their body.” And perhaps to stay in tune with ourselves, it is worth remembering the complex changes throughout the body that occur beyond what we feel at any moment.
“Your experiences in life may make you feel sleepy,” says Lucas, “but underneath that there are a lot of things your body needs to prepare for. The same thing happens when you're hungry, frisky, etc.”
