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🕤️ What happens to your sleep if you live underground for a month?
Kevin
February 24, 2024
This week on the Sleepstack Newsletter:
• 🌚 What locking yourself in a cave for a month does to your sleep
• 🏋️ How to time your sleep cycle for peak athletic performance
• 🔋 The best ways to reset your body clock (German accent helpful)
🛌 They camped in a dark cave for a month. Here’s what happened to their sleep…
Nathaniel Kleitman (left) is considered a father of sleep research. Together with his student, Bruce Richardson, they camped out in Mammoth Cave, Kentucky to study the sleep-wake cycle.
For about a month in the summer of 1938, and Professor Nathaniel Kleitman at the University of Chicago, and his student Bruce Richardson, performed an astounding feat of scientific dedication.
Loaded with food and water, several measuring devices and a pair of high-standing hospital beds, they camped within a small rock chamber deep inside Mammoth Cave in Kentucky, one of the deepest caverns on the planet.
 Historic entrance to Mammoth Cave. Source: nps.gov | Far underground the cave was entirely devoid of natural light and maintained a fairly cold temperature around 10-15 degrees Celsius. |
They didn’t just manage to grow some impressive facial hair though - they also made some groundbreaking discoveries in the process. The first was that humans can generate their own sleep-wake cycle in complete darkness. Kleitman and Richardson didn’t sleep and wake in random spurts… instead they seemed to lock into a repeating pattern of wakefulness (about 15 hours), followed by sleep (e.g. 9-10 hours).
But they made a second, even more surprising discovery: their natural repeating cycles of wake and sleep were not actually 24 hours in length, but consistently longer than that! Richardson, in his 20s, developed a sleep-wake cycle of between 26 and 28 hours in length. Kleitman, in his forties, was slightly over 24 hours. It turns out that when removed from the external influence of daylight, our internal body clocks don’t run for exactly 24 hours - but a little more than that!
In the decades since Kleitman and Richardson’s cave-dwelling experiment, we have now determined that the average duration of a human adult’s endogenous body clock runs around 24 hours and 15 minutes in length. Today we call this the “circadian rhythm”.
🏅 A healthy body clock maximises performance, but don’t get out of sync!
It’s not a coincidence - your chances of breaking an Olympic record are clearly linked to time of day.
Your circadian rhythm helps determine when you want to be awake and when you want to be asleep. But when it’s out of whack it can have far-reaching effects on your eating habits, mood, body temperature, metabolism, hormone release and more.
On the other hand, a healthy and regularly-reinforced sleep-wake cycle can help boost performance and metabolic response when exercising at certain times of day. Studies show that peak athletic performance, coincides with the body's natural circadian peak in the early afternoon. This explains why Olympic records are most often broken between 12 and 3pm!
Even the timing of births and deaths follows circadian patterns, reflecting the rhythm's impact on essential metabolic, cardiovascular, and hormonal processes.
⏱️ Zeitgebers: the best ones to recalibrate your body-clock
Thankfully, most of us don’t live in Mammoth Cave, or constant darkness. Our body have ways of regularly adjusting our internal clock, to keep it ticking along smoothly. Any signal that the brain uses for the purpose of clock-resetting is called a zeitgeber, from the German “time-giver” or “synchronizer.”
Light is the most reliable and therefore the primary zeitgeber. Our brain and nervous system responds especially to the wavelengths of light that occur in the first few hours of the day. Morning sunlight methodically resets internal rhythms every day, “winding” us back to precisely, not approximately, 24 hours.
Catching sunrise over the lake - Heslington, UK.
Daylight is the most reliable, repeating signal that we have in our environment. Since the birth of our planet, and every single day thereafter without fail, the sun has always risen in the morning and set in the evening.
While daylight isn’t the only signal that the brain can latch on to - it is the principal and preferential signal for resetting the biological clock!
But so long as they are reliably repeating, the brain can also use other external cues as zeitgebers - such as food, exercise, temperature fluctuations, and even regularly timed social interaction. All of these events have the ability to reset the biological clock, allowing it to strike a precise 24-hour note.
This is why people with certain forms of blindness do not entirely lose their circadian rhythm. Despite not receiving light cues, other phenomena can act as their resetting triggers.
⚡️ Recommendation zone
❗️Actionable insight: Use morning sunlight to reset your body clock
Morning light, especially the first natural light of the day, is a powerful wake-up signal because of the unique ratio of wavelengths that appear at sunrise.
Spending 5-10 minutes outside soon after waking up (on even the cloudiest of mornings) can trigger a powerful synchronisation cue for your internal clock and is linked to numerous health benefits.
🛠️ My current Sleepstack:
Screen filter software: Tools like f.lux (all desktops), Twilight (Android) and Night Shift (iOS) change the colour of your display to based on the time of day to reduce overstimulating wavelengths.
Sleep mask: I use this one at the moment. Light affects melatonin secretion even when your eyes are shut. A good sleep mask is life-changing, I’m never going back.
That’s all for this week!
We’ll be back in your inbox on Saturday,
Kevin
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