The evolutionary reasons for sleep are a mystery. Even top biologists concede they are not sure why we sleep from an evolutionary perspective, although there are plenty of hypotheses. Why did Mother Nature invent sleep and why did it persist over millions of years? All animals sleep and some of the mechanism involved in human sleep seem to go back to before mammals appeared on the earth. Although many good and necessary functions happen in the body while we sleep, it is not obvious why these functions should occur or must occur during the sleep stages. The downsides of sleep to survival – exposure to predators and less awareness of our surroundings for hours every night.
Primitive organisms exhibit circadian rhythms and go from periods of activity to rest. Sleep in higher animals probably arose from the basic rest periods, and became more structured and the body learned to carry out specific functions during sleep. Sleep takes up such a large portion of the day for most animals that it has been caught up with the evolution of every species and the ecology of entire ecosystems.
Some of the physiological structure of sleep spread across many orders of the animal kingdom. Evolutionary scientists take this as evidence that sleep is very old in an evolutionary sense, long predating the rise of mammals. The hormone melatonin, so important in human circadian cycles, plays a part in behavioral regulation in many organisms, even worms. Zebrafish larvae have been used to screen insomnia drugs for humans. This screening isn’t an observation of the physiology of sleep (e.g. brainwaves) but of the behavior of the animals.
The sleep patterns of different species has led to the idea that sleep evolution in mammals and birds has been shaped by chasing prey and fleeing from predators.
It cannot be a coincidence that small animals are more likely to sleep in shorter bursts. The waking periods allow them to check for threats. Just among mammals, there is a wide range of sleep times. Bats can sleep over 20 hours per day while horses get by on four or less. Some animals are nocturnal and some are diurnal. Most mammals display some sort of polyphasic sleep pattern – alternative periods of sleeping and waking. Even though their sleep is spread among more than one period, it may not be evenly distributed over the course of the 24-hour day. This is why is makes sense to talk of animals that are nocturnal (active at night) and (polyphasic sleep in multiple periods)
Generally speaking a survey of mammals finds:
Smaller mammals have a polyphasic sleep pattern. At one time, biologists thought this could be an evolutionary adaptation to environmental risk or predators. By avoiding long periods of sleep, they could be reducing their danger of being attacked while unconscious. This is speculative, though. Animals (and people) often briefly awaken in the transition between NREM and REM sleep (you may not remember it in the morning). This time gives the animal a brief period of consciousness to assess risk. It has therefore been thought that shorter sleep cycles may have evolved in animals subject to predation.
However, analysis of data has shown these hypotheses to be false. Small animals do tend to sleep in a polyphasic manner, but there is no real correlation with the phasing and duration of sleep with risk for being eaten. It appears more probable that small animals need to sleep polyphasically to wake up and eat.
The evolution of sleep is caught up with the evolution of brains.
Many simple cold-blooded animals control waking from the brainstem, which is a simpler brain than mammals have. Mammals control waking from the cortex. The more primitive waking period was converted to slow-wave sleep, scientists speculate. The old style sleep of primitive animals was converted to REM sleep. This explains why mammals have such complex sleep. The evolutionary leap from amphibians to humans didn’t result in acquisition of a new way to sleep; it resulted in a new way to be awake.
Scientists feel that REM sleep, or a precursor to REM, started before the arrival of mammals, and originated in reptiles. Mammals and birds do not have much thermoregulation during REM, and cold-blooded reptiles do not have much thermoregulation period. Non-REM sleep in mammals and birds involves activity in the cerebrum, a part of the brain underdeveloped in reptiles. So it appears that REM is a vestige of an evolutionarily early form of sleep.
The body and brain do many functions during the sleep period and the traditional idea was that sleep evolved to serve these functions. Call this the “adaptive theory”.
More and more evolutionary biologists are returning to the idea that sleep primarily started as a way for the animal to rest, conserve energy, and be still to avoid predators. This seemed an overall simplistic explanation at one time. After all, so many important brain maintenance functions happen while we’re asleep that it must be more than that.
The thinking in this new model, however, involves the evolutionary concept of exaptation. Exaptation is a term biologists use to describe how a species (or genetic line of organisms) repositions functions of traits over the generations. A behavior or anatomical feature may exist in a species but over time come to serve a function other than its original function. Sleep may have started simply to conserve energy and to prevent animals from moving around when it was dangerous to do so. Over millions of years, other functions were added to the sleep period. Consolidation of memories, for instance, is probably easier for the brain to accomplish during an unconscious period, when the rate of new memories being formed is low. The same is true for the flushing of the glymphatic system.
This story parallels evolutionists; ideas of how other complex systems and behaviors developed. This is well explained by Scientific American writer Boris Zivkovic in an article about the “non-adaptive function of sleep.”
Primates sleep uniphasically – the whole day’s period is in one sitting – or perhaps biphsically with a short waking period between the two sleep periods. This is one way we differ from other mammals. Aquatic mammals – which must come to the surface to breath air – have evolved a way to sleep in one half of the brain at a time. This is called unihemispheric sleep. The waking half keeps the dolphin or whale moving and breathing while the other half sleeps. Their brains have a special anatomical feature to prevent both halve from sleeping at the same time.