I’ve heard several scientists say that we still don’t really understand the purpose of sleep. In their TED talks, Russell Foster and Jeff Iliff propose some possible explanations. Foster’s ideas sound like mere common sense, while Iliff’s suggestion is more interesting: He claims that during sleep, waste products are purged from our brains. In other words, your brain poops while you’re sleeping.
That may be true. But I have my own theory. It’s kind of a joke, but deep down, I think there might be something to it. It’s something that any parent will recognize as an almost certain truth. And it’s simple: Sleep evolved so that little kids would periodically shut down, leaving parents free to do the things that they need to do in order to keep their family alive.
Consider what happens when you have a baby: You’ve got this tiny, helpless thing that screams incessantly, thereby attracting predators that Mommy and Daddy have to keep fending off. After a while, this thing starts crawling all over the place — into thorn bushes and ant hills, and over the edges of cliffs — so you have to spend all of your time running around protecting it from self-inflicted injuries. And on top of that, the thing keeps putting everything into its mouth. Constant vigilance is required to ensure that your baby doesn’t choke or poison itself.
Under these circumstances, a mother and father living on the high plains will never have time to do anything. They won’t be able to gather food, cook a meal, build a shelter, make clothing, start a fire, or even take a dump without their baby going off and getting itself killed. So all the people whose children don’t periodically shut down will be weeded out by natural selection. Of course, this must go back much earlier than humans. Our shrew-like ancestors also would have needed time to hunt, gather food, find shelter, and so on. And so they would have needed for their children to sleep, too.
If my theory is correct, one must ask why adults also need sleep. But it could simply be that an adult’s need for sleep is just a remnant of what was vital in childhood, or that sleeping as an adult provides other benefits, as described in the brain-poop theory. What’s telling is that kids need so much more sleep than adults, thereby giving their parents time to get shit done.
So, how’s that for a theory? I think there might be something to it. Maybe sleep is mostly about recharging, getting rid of waste, and conserving energy. But perhaps there was also some evolutionary pressure for children to sleep longer hours than their parents and to nap during the day, not for the sake of recharging or conserving energy, but for the sake of giving their parents time to do what was necessary to keep the child alive.