How and Why Sleep Evolved

Sleep. We all need it, and most of us don’t get enough of it. And even if we get the average recommended amount (the good old 8-hour dosage), that takes up about a third of our lives overall.

But why do we sleep, from an evolutionary standpoint? The fact that it’s so widespread in the animal kingdom attests to it having some kind of vital function. But it just seems like a waste of 8 hours, more or less.

Why do we sleep?
Avoiding sleep can even be downright crushing for your health.

We would certainly benefit from the time normally spent sleeping – just imagine how more productive we would all be. We could be off working, eating, reproducing, and who knows what else. And while we’re off to see the Sandman, we leave ourselves completely open to predators or other dangers.

But we nevertheless require sleep. Avoiding it can even be downright crushing for your health – and we often resort to waking nights in our modern worlds. And although there are technological solutions for improving sleep, there exists no way to make it unneeded.

So what’s the advantage that sleep brings to the table?

How We Sleep

Before we get to the why, we should first understand exactly how sleep works. And don’t worry, it won’t get too technical – this will be more of a broad overview of the process.

The brain has a certain sleep rhythm which it prefers (this is why we prefer sleeping at night). The mind uses the information it receives from its eyes to help it decide when it’s the proper time to shut down, so to speak.

When you finally hit the hay, the thalamus, which serves for the intake of sensory input, turns off for most of the night. This lets you tune out from external stimuli.

Sleep is divided into four stages, which alternate multiple times each time you sleep:

  • Stage 1 non-REM is the transition between wakefulness and sleep. It normally lasts just a few minutes, and everything in the body slows and unwinds: the muscles, the heartbeat, eye twitching, breathing, as well as your brainwaves.
  • Stage 2 non-REM is the step before deep sleep and the one we spend the most time in while asleep. Your temperature drops and bodily activity slows down even further. The brain also continues its slowing but is marked by occasional spikes in electrical activity.
  • Stage 3 non-REM takes us into a deep sleep, and waking us up from it becomes more difficult. It happens mainly in the first half of our sleeping session, and our breathing, as well as our heartbeat, hits its slowest pace. This is the stage we need the most to feel recharged upon awakening in the morning.
  • REM sleep takes about 90 minutes to begin. It’s easily recognized by rapid eye movement left-to-right behind closed eyelids (from which REM gets its abbreviation). Your brain (like the thalamus) spark back up, bringing back your body activity to near-awake levels. The bulk of your dreaming unfolds in this stage, and your muscles become paralyzed, so as to prevent you from moving in your sleep.

Why We Sleep

The truth is that we simply aren’t sure why humans – and the vast majority of all animal classes – sleep. However, we do have a couple of educated guesses.

Sleep as Psychological Maintenance

One widely popular theory posits that sleep is the brain’s method of sorting out its neural connections. Namely, when we sleep, the brain will decide which neural connections it wants to reinforce. This does explain why we form more solid memories and muscle memory skills after we’ve had a proper amount of sleep.

Meanwhile, another theory claims that the purpose of sleep is to discard the connections that it doesn’t need. We take in a potentially infinite amount of information every second, and we filter out most of it. Some unnecessary info does stick around for a while, though, and the brain relegates those to the cutting floor.

While certainly seeming sound, these theories aren’t exactly watertight. For one, we lack the empirical evidence to solidly prove either of the two. Secondly, even from a hypothetical perspective, some outliers create loose ends.

For example, why does the big brown bat sleep for 20 hours a day? Or why do American black bears hibernate for months at a time? Even some dolphins refrain from sleeping for weeks during migration.

Sleep as Adaptive Behavior

Beyond the idea that sleep is a kind of brain-cleaning, other experts believe that sleeping is actually evolution’s way of protecting us from harm.

More precisely, they claim that sleep is an adaptive behavior that serves to keep us out of harm’s way or prepare us for when danger is at hand. It may not add up initially, but it makes sense when you give it some more thought.

Consider the case of the big brown bat we mentioned earlier, which is only active for 4 hours at a time. That means that for these 4 hours it flies around catching specific kinds of insects that are only active in that same time window.

And during these four hours, it becomes exposed to predatory birds praying on it. But when asleep, it’s well hidden from any such threat. In addition, it doesn’t burn calories when it doesn’t need to, preserving energy for when it is required.

Now the picture becomes a little clearer. Animals (and humans, back when it mattered more) used sleep to keep themselves pinned to a safe place. That way, they won’t wander off somewhere and risk getting eaten. The torporific state also prevents us from moving, thus keeping our caloric output minimal, so that we may have enough energy to perform important tasks.

So, thus far we have no crystal-clear consensus on the purpose of sleep. However, we are well-aware of the fact that it is paramount to good health. To that end, make sure you get enough sleep and look into solutions for fixing any kind of sleep disorder you might be experiencing.

Dr. Nikola Djordjevic, MD

I'm a board-certified family physician, co-founder and project manager of MedAlertHelp.org. I also serve as a medical advisor for the wonderful authors at DisturbMeNot. We work together on providing you with the latest and most accurate information regarding sleep health and beyond.
Dr. Nikola Djordjevic, MD

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