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- The two separate biological mechanisms that result in sleep
- How and why the “pressure to sleep” is generated
- The role the biological clock plays in regulating the body’s functions
Understanding what makes us sleep and how it happens begins with two separate biological mechanisms: sleep-wake homeostasis and the circadian rhythm. These two mechanisms have their own functions, but their interplay results in your wakefulness and sleepiness.
Sleep-wake homeostasis generates an increasing pressure to sleep over the course of the day.
The circadian rhythm generates a decreasing drive for alertness over the course of the day.
The circadian rhythm combats the pressure from the sleep-wake homeostasis mechanism until a window in the evening called the “sleep gate”. During this window, the pressure to sleep is far greater than the alertness generated by the circadian rhythm eventually resulting in sleep.
”"I love sleep. My life has the tendency to fall apart when I'm awake, you know"Ernest Hemingway
How sleep-wake homeostasis works
The “pressure to sleep” generated by sleep-wake homeostasis is a sleep-regulating substance called Adenosine. Adenosine is a nervous system depressant produced as a byproduct of cellular power production.
The longer you stay awake, the more Adenosine builds up in your brain. The greater the buildup, the more you desire sleep.
When you sleep, Adenosine is flushed from the brain and you wake up with less of a desire for sleep (despite that urge to press the “snooze” button). When you don’t sleep enough, the Adenosine hasn’t been completely cleared from your brain.
Imagine a cleaning crew coming into your office. This cleaning crew needs about 7 to 9 hours to clean everything but for the past week or two, they’ve only been given 5 or 6 hours to do the same amount of work. This means areas are left uncleaned, tasks are left undone, and the office gets messier everyday it’s used.
The office mess is the lingering Adenosine in your brain. It makes you feel groggy and fatigued, getting worse with continued sleep restriction.
Just like a single opportunity to clean for 8 hours isn’t enough to deal with the built-up office mess, a single night of good sleep isn’t getting rid of your fatigue. When you don’t get adequate sleep, you develop a “sleep debt”. It takes time to catch up on the missed sleep as the functions of the body need to deal with extra clean-up, as well as the continued daily buildup.
The key takeaway? We need 7 to 9 hours of sleep consistently and “catching up” when you miss a few hours takes more than a single night of good rest.
How the circadian rhythm works
The circadian rhythm is a 24-hour cycle timed by the Suprachiasmatic Nucleus (SCN), your body’s biological clock. Using dedicated light receptors that go directly to the optic nerve, the SCN adjusts the body’s clock according to the day/night cycle. This rhythm regulates our metabolic, physiological, and behavioural functions, like hormone release, body temperature, feeding patterns, cell regeneration, and more.
Like Adenosine is for sleep-wake homeostasis, Melatonin and Cortisol are the most important hormones for the circadian rhythm.
Melatonin causes drowsiness, and Cortisol promotes wakefulness.
Melatonin levels increase in the evening and continue through the night. Once awake, Melatonin quickly decreases. Cortisol levels are typically lowest in the evening and rapidly increase in the morning. Cortisol is commonly known as the “stress hormone” activated during the fight-or-flight response (exactly when you need a boost in alertness).
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