Chapter 4 Does Every Species Sleep?
Short answer: yes
Really?
Yes, even some of the very simplest forms of unicellelar organism , such as bacteria, have active and passive phase that correspond to the light-dark cycle of our planet. It is a pattern that we now believe to be the prescursor of our own circadian rhythm, and with it, wake and sleep.
Just today (01-08-2025), I saw an article saying that a new creature was discovered with a sleep-like state. And the surprising part—this creature is one we’re all familiar with: the jellyfish, a brainless animal. source: https://www.nature.com/articles/d41586-026-00044-7
Sleep so important
In fact, it is so important that we might ask ourselves this question - why do we even bother to wake up? Maybe sleep was the first stage of life on this planet, and it was from sleep that wakefulness emerged.
The true question: how?
To answer this question, we can break it down to 1. Duration 2. Composition 3. Ways of sleep
1. Duration
Elephants need half as muc hsleep as humans, requiring just 4 hours of slumber of each day. Tigers and lion devour 15 hours of dailly sleep. The brown bat outperforms all other mammals, being awake for just 4 hours each day.
You might think species in the same phylogenetic specis share the same sleep time? Answer: no
Squirrels and degus are part of the same rodents family group. Squirrels need twice as long as the rodents – 15.9 hours.
It seems there is no one factor that decides the duration of sleep. Instead, it involves a complex hybrid of factros, such as dietary type, predator/prey balance within a habitat and many more.
Conclusion: Everybody sleeps differently in terms of duration. Some speices are simply big sleeper and some are just more of a hustler 🥸
2. Composition - In terms of NREM and REM
Every species in which we can measure sleep stages experience NREM sleep. However, insects, amphibians, fish, and most reptiles show no slear signs of REM sleep. Only birds and mammals (excludes ocean mammals), which appear later in the animal kingdom, have full-blown REM sleep.
3. Ways of sleep
Ocean mammals, such as dolphins and whales, their sleep are unihemispheric. It means they sleep with half a brain at a time! They do this becasue they need at least half of the brain to stay awake to maintain life-necessary movement in the aquatic environment.
Birds, when alone, do the same thing - with half of brain sleep and the other half awake. However, when birds group together, the flock will line up in a row. The birds on the two ends of the line will sleep with half brain, but the rest can indulge full brain sleep!
How does Human Sleep?
Homo erectus, the predecessor of Homo sapiens, is believed to be the first human species to consistently sleep on the ground. Before that, our ancestors slept in trees for evolutionary reasons (Trees kept them safe from predators), but let’s be honest: it probably wasn’t great for sleep quality. Imagine falling out of a tree mid-dream. Not ideal.
So what changed?
Homo erectus is widely believed to be the first to control fire. Fire scared off large carnivores and made it safe to sleep on the ground. For the first time, humans didn’t have to worry about gravity betraying them in their sleep. Better safety meant better REM sleep, which likely accelerated brain complexity and neural connectivity.
In other words, sleeping on the ground (thanks for fire) may have triggered a revolutionary leap that helped make us smarter.
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Last Modified: 2026-01-08