Everyone’s got one, and it’s what makes the circadian rhythm tick.
It’s an internal body clock, called a circadian clock, which uses a roughly 24-hour rhythm that coordinates with the Earth’s light-dark cycle. It tells your body when to sleep and when to wake.
Known as the circadian rhythm, it has two components — wakefulness and sleep.
The circadian rhythm follows a predictable day-night pattern every day, made possible by external cues such as sunlight, temperature, and controls whether you feel tired or alert.
Your internal 24-hour body clock, located within your brain, communicates the daily circadian rhythm to every other region of your brain and organs. It helps determine wakefulness and sleep, preferred times to eat and drink, core body temperature, moods and emotions, and the release of various hormones.
Your 24-hour biological clock
The 24-hour biological clock is called the suprachiasmatic nucleus (SCN) and sits in the center of your brain. It “samples” the light signal being sent from each eye along the optic nerve and resets any time inaccuracy of the 24-hour cycle.
During the seasons, as days get shorter or longer, or for time-zone travel, it’s the light from the sun that resets our internal clock back to precisely 24-hours.
The suprachiasmatic nucleus also controls many behaviors and, of course, the circadian rhythm that regulates sleep.
The circadian rhythm explained
It’s easy to consider the 24-hour circadian cycle in distinct stages, such as when you wake up in the morning, through to when you can’t stay awake any longer at night, and then sleep.
Wakefulness and sleep, two primary components of the circadian rhythm, repeat every 24-hours, this being the circadian cycle.
During the cycle, all sorts of things are happening to your body, with the key ones discussed below.
The rise and fall of core body temperature
Just to get things straight, your “core” body temperature is your rectal temperature, and during the 24-hour circadian rhythm, it rises and falls.
Indeed, a drop in the core body temperature of 1-degree is one of the keys to going to sleep.
Around 6 pm, your core body temperature starts to drop from its peak of around 37.1 degrees and continues dropping until it reaches its lowest point of about 36.7, about 2-hours after sleep. Then it slowly starts rising again, a few hours before you wake up.
Your circadian rhythm is responsible for the core body temperature to drop by 1-degree as bedtime approaches. The drop in core body temperature is one of the essential factors in getting to sleep. Likewise, your core body temperature increases a few hours before waking up. It’s a demonstration of one of the many 24-hour rhythms the suprachiasmatic nucleus governs.
The rise and fall in core body temperature repeat every 24-hours, regardless of whether you’ve slept or not.
Melatonin helps regulate the timing of when sleep occurs
Melatonin is a hormone produced by the pineal gland in your brain a few hours after dusk.
The purpose of melatonin is to tell your brain and body that nighttime has arrived and sleep is not far away. This is how your 24-hour biological clock lets your body know that darkness has come.
From that moment after dusk, the melatonin in your body rapidly increases until you fall asleep, with peak concentration occurring around 4 am. Then the level of melatonin decreases throughout the night, and by dawn, as sunlight occurs, the pineal gland shuts off the production of melatonin.
When sleep is over, wakefulness begins.
What’s important to note is melatonin helps regulate the timing of when sleep occurs. It doesn’t generate sleep.
Sleep pressure and the urge to sleep
Sunlight! You’re awake, refreshed, now for part two of the circadian cycle.
Part two is called “sleep pressure.” It starts as soon as you’re awake and builds throughout the day.
The cause of sleep pressure is a chemical called adenosine, which continually builds up in your brain while you’re awake. The longer you stay awake, the higher the concentration of adenosine.
Adenosine has one essential function. The greater the buildup of adenosine in your brain, the greater the need to sleep — hence the expression, sleep pressure.
Sleep pressure builds to such an extent that there is an overpowering urge to sleep for most people after being awake for 12–16 hours. You simply can’t stay awake any longer and must sleep.
There’s an exception: coffee or caffeine, which can keep you awake, even when you need to sleep. Caffeine does this by artificially muting the adenosine’s sleep signal, thereby making you feel more awake and alert than you otherwise would be.
Without coffee and once asleep, the brain removes the day’s adenosine, and after a healthy 8-hour sleep, the adenosine is wholly gone. And then, the whole process of sleep pressure repeats the moment you’re awake.
Melatonin and adenosine are not linked
The two governing forces of the circadian rhythm are melatonin and adenosine.
On the one hand, there’s melatonin telling the body that darkness has arrived and get ready for sleep. And on the other there’s adenosine, which keeps building in concentration throughout the day, causing sleep pressure and eventually the urge to sleep.
Both these chemicals may appear to be collaborating or communicating with each other. But they’re not linked. They’re just in step with each other.
As strange as it may seem, the circadian rhythm pays no attention to whether you’re awake or asleep. It just reliably repeats itself every day.
In the morning, the circadian rhythm begins to increase its activity a few hours before you wake up, instilling the brain with alerting signals. By early afternoon, the activating signal from the circadian rhythm peaks and then declines until it reaches the lowest point a few hours before you awake.
This cycle happens every single day, regardless of the amount or lack of sleep you have.