Ever wondered why jet lag hits so hard, or why so many people seem to be fighting a quiet battle with sleep deprivation? It's the same quiet mystery behind another puzzle: why does a night-shift worker feel drowsy the moment their shift changes, almost as if the body has its own complaint about the schedule?
And then there are the nocturnal animals, the ones that roam out at night and sleep through the day. None of this is random. All of it points to something deeper going on inside us, a clock that starts ticking long before the alarm in the morning.
Your body runs on its own clock
We keep clocks around the house to know the time and plan out our day. In a similar way, our body carries a master clock, one that runs around the clock, twenty-four seven. It keeps a certain rhythm, and through that rhythm it manages our sleep-wake cycle for us. That rhythm has a name: the circadian rhythm.
Naturally, the next question is how it works. Who tells this clock what time it is, so it knows exactly how to steer our wake-sleep cycle?
More than just sleep
Here's the part I find remarkable. Circadian rhythm applies to all living beings, and it doesn't stop at sleep. It also holds sway over cellular, behavioural, psychological, hormonal, fertility and metabolic aspects of life, among many others. In a way, it runs the whole show behind the scenes, quietly and without asking for credit.
A host of factors can influence it: light, temperature, oxygen levels, steroids, how we ingest hormones, physical activity, lifestyle, stress and age, to name a few. Light plays a particularly important role here, and the light-dark cycle has a distinct impact on the rhythm. Think about how a dark room makes you drowsy, or how bright morning light wakes you up more quickly than any cup of tea.
The two hormones in charge
Two hormones largely regulate our sleeping and waking: melatonin and cortisol. Put loosely, melatonin is what makes us drowsy, while cortisol keeps us alert. The level of each shifts throughout the day for every living being, rising and falling on its own schedule.
Those levels look different for babies, teenagers and older people. There's a reason babies run on such an erratic sleep cycle, for instance: their circadian rhythm simply takes time to develop, and until it settles, a nap is never far away. And older people tend to feel sleepy a little earlier than teenagers, partly because of how these hormone levels change with age.
Why jet lag happens
Your body adjusts to the time zone you're currently in and manages its rhythm accordingly. That's why you feel disrupted when you travel somewhere with a different time zone: the biological clock needs some time, often a few days, to realign with the new cycle of light and darkness around it. The jet lag you feel after a long flight is really your body's hormones, temperature and sleep drive all being temporarily out of sync with the world outside the window.
Here's the part I find most reassuring about all of this. The circadian rhythm doesn't need you to think about it. It just runs, quietly coordinating everything from when you feel hungry to when your body temperature dips at night. All you have to do is give it the right signals — a bit of morning light, a consistent bedtime — and it rewards you by keeping the whole system humming along.
So the next time you feel groggy on a long-haul flight or a little out of sorts after a shift change, blame the master clock, not yourself. Your internal rhythm is only catching up — and once it does, it quietly gets on with running your day without you even noticing.