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We all do it. Well, all of us humans do it. When it comes to other organisms, it gets complicated. Actually, it’s complicated for humans, too. This episode, we discuss the whos, hows, and mysterious whys of Sleep.
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Zzzzz
Defining sleep is trickier than it seems at first. Sleep isn’t quite the same as simply resting or falling unconscious or even hibernating.
Sleep is a distinct behavior with certain consistent features. A sleeping animal is mostly inactive, generally assumes a particular sleeping posture (we humans lay down, for instance), and is difficult to rouse (although not as difficult as, say, during a coma). Sleeping animals also exhibit particular changes to their metabolism, brain activity, and muscle activity that can be recognized by special equipment.
The human on the left is unaffiliated with the podcast.
But defining sleep gets even trickier when we consider all the variation we see among animals. Some animals sleep most of the day, while others sleep for just a few hours per day. Some animals are partially active while they sleep, keeping an eye open for threats or even still moving around their environment. Some animals, like marine mammals and certain birds, sleep with half of their brain at a time! Sleep-like behavior has been observed in animals as simple and strange as jellyfish (which don’t even have brains) while in many animals, sleep is so hard to identify that we’re not quite sure they truly sleep at all. And many animals change their sleep schedule at different times of the year or during different phases of their lives.
Despite all of this variation, sleep is clearly important. In humans as well as other animals, lack of sleep causes all sorts of problems, including loss of cognitive functions, and if we don’t get enough sleep, we will need to sleep extra long to make up for it. It would be much easier – and safer for many animals – to simply not sleep at all, but that doesn’t seem to be an option. Which raises the question: what is sleep for anyway?
Right: A duck in resting position. Image in Public Domain.
The question of why we sleep has been discussed and debated intensely for a long time. It seems likely that sleep offers the body and brain a chance to refresh for a new day, but exactly what is happening during sleep is difficult to explore. It might be that our brains are clearing away built-up waste or repairing damage; perhaps it’s important to lower our body activity regularly to prevent overworking; maybe there are cognitive benefits, that sleep somehow helps us to organize our thoughts or memories. All of these and more hypotheses have been put forward, most of them have been debated, and few or none of them are fully agreed-upon by experts.
Sleep is such a common behavior among animals that it’s likely something that evolved very early on. It’s possible that sleep is a side effect of our circadian rhythm: the tendency for animals’ activity levels to vary throughout the day. It would certainly be beneficial for a nocturnal animal to rest and conserve energy during the day (and vice versa), so perhaps the body eventually evolved the habit of using that period of inactivity to accomplish … well, whatever it is that sleep is accomplishing.
These are very much questions under active investigation. The more we study animal life – and ourselves – the closer we get to unraveling these unexpected mysteries about the evolution, origins, and functions of sleep.
Learn More
What happens if you don’t sleep?
The origins and evolution of sleep, 2018 (technical, open access)
Clues to the function of mammalian sleep, 2022 (technical, open access)
Evolution of Sleep and Adaptive Sleeplessness, 2019 (technical, paywall)
Mei long, the dinosaur caught napping
Hibernation is different from sleep
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COMMON DESCENT 
Another fabulous episode guys, thanks. Just thought I would share this related tale, I have one of those Perspex bird feeders outside my kitchen window and there are usually three or four birds on it at a time but recently a goldfinch landed on it (in the middle of the day), tucked his head under his wing and went to sleep, and I was intrigued to see that all the other birds (of many types) let him be. A couple landed on the edge and then retreated and left him to have his nap in peace. As soon as he woke up and departed feeding resumed as normal. I thought that was pretty amazing at the time.
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