Circadian rhythms have been around longer than sleep. BOOM! (Krueger, 2020)
That was perhaps one of the best comments in a recent review article. Because the earth and sun were around long before any animals and it's unlikely the earliest life forms "slept."
Whatever it's purpose, it'd better be damn important! Because you're basically unprotected and vulnerable to easy attack. A lucky animal's free lunch or get assassinated by a neighboring tribe.
Indeed, the unresponsiveness of sleep must be a big deal.
Chronologically, circadian rhythms helped to create life and shape it as well. It's unknown when sleep came into the picture, and although it's unlikely that plants sleep now, that doesn't mean they won't evolve something similar in the future. I don't know.
One of my favorite theories as to the purpose of sleep is neuronal plasticity/connectivity. Like, the only way we could learn stuff would've been one of the most or THE most important thing, thus justifying the immobility and unresponsiveness of sleep.
Although I'm not sure of the timeline here - eg, how did we learn to avoid predation while sleeping? It's kinda confusing through that lens.
Also, death by insomnia is a thing. It'll kill you faster than starvation.
More and more studies are showing deleterious effects of circadian arrhythmia. Shift work, jet lag, and mistimed meals won't get you as fast as sleep deprivation or starvation, but they'll get you eventually (Leung and Martinez, 2020). You'll see it listed as CVD, cancer, infection, etc., but there's a strong probability of a circadian arrhythmic component.
:: a little non sequiter ::
Sunlight enters your eye from above you, and this is the most effective way to suppress melatonin. It's more effective than from experimental conditions where blue light is shined in your eye from a lower angle.
The moon (light from above the eye) and bonfires/campfires (light from below) don't suppress melatonin. Having a bonfire may have been a good deterrent to permit the immobility and unresponsiveness that I think is required for sleep's neuronal plasticity/connectivity function. Therefore it's good that bonfires don't suppress melatonin (and moonlight). Coincidence?
What's your personal or favorite theory on the purpose of sleep? Leave a comment!
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Get some dark chocolate, too :)