Sleep in Animals
Sleep is present in all animal species, even invertebrates. And bacteria that survive for longer than 24 hours have circadian-like rhythms. As we’ve said before, this suggests that it’s universally critical for survival.
We ask all the time why we sleep. One researcher posed an interesting inversion of the question - if wakefulness is damaging to the body and sleep recovers it, why did life ever bother to wake up? (Shortform note: of course, you can’t be productive and reproduce when sleeping, so sleeping too much would be evolutionarily disadvantageous.)
The amount of sleep per day varies from 4 hours in elephants to 19 hours in bats. There are no strong correlations between animal characteristics and amount of sleep, though within animals of a similar size, a more complex brain increases sleep.
Among animals, REM sleep occurs only in birds and mammals. Because REM evolved independently in these two distant evolutionary trees, REM likely performs a critical function that NREM cannot accomplish, or that REM is more efficient at accomplishing.
Interesting animal sleep patterns
- Cetaceans (dolphins, whales) sleep with half their brain at a time. They also don’t have REM sleep (as formally defined), since the muscle atonia would prevent swimming. But they may have some variant of REM that’s harder for researchers to detect.
- Birds in a flock will have most birds in full-brain sleep, then birds in the perimeter sleeping with half their brains to stay alert for danger
- Similarly, humans in a new environment (like a new hotel room) show one half of the brain sleeping lighter than the other half, like it’s staying alert to detect danger. This is why the first night in a new environment can be so unrestful. As you acclimate to the environment, this half-brain sleep dissipates.
- Transoceanic birds that cross thousands of miles have ultra power naps, sleeping for seconds at a time.
Sleep in Humans
What’s the ideal human sleep pattern? Native pre-industrial tribes show a biphasic sleep pattern, with 7-8 hours at night and a 30-60 minute nap in the afternoon. At night, they sleep 2-3 hours after sunset, awaking around dawn.
- All people tend to have a dip in energy in mid-afternoon. (So if you’re scheduling a presentation, avoid the slot right after lunch.)
- A study of Greek siestas showed that people who abandoned siestas (due to political and social pressure) showed a 37% increase risk of death from heart disease compared to those who maintained siestas.
What about supposed historical styles of sleeping, like segmented sleep (two periods of sleep at night, separated by a few hours of wakefulness)? The author argues this is mostly a cultural artifact, and not a natural way to sleep. No evidence suggests a biological desire to wake up for a few hours in the middle of the night
Relative to great apes, humans sleep less (8 hours in humans vs 10-15 hours in apes) and have more intense REM sleep (20% in humans vs 9% in apes). Matthew Walker hypothesizes this evolved as follows:
- Apes sleep in trees and enjoy great safety at night.
- In the human ancestor Homo erectus, an upright body posture and shorter arms made sleeping in trees more difficult. REM sleep is also dangerous in trees because limp muscles increases the risk of falling out.
- Discovery of fire allowed humans to ward off predators and parasites at night. But danger still inevitably lurked, so humans who could sleep more efficiently for less time were evolutionarily selected for.
- Then as human societies became more complex, REM sleep became more important. REM sleep is found to be critical for 1) internal emotion regulation, 2) reading emotions from others, 3) creativity. This led to improved survival strategies and larger social groups, which further increased brain complexity and more need for REM sleep, forming a positive feedback loop.