We spend a third of our lives asleep, and science still does not have one solid answer for why

Sleep is evolutionarily expensive — while asleep, an animal isn't hunting, isn't fleeing predators, and isn't reproducing — and yet it shows up in nearly every animal ever studied, from humans to fruit flies, suggesting it must serve some genuinely critical function.

Leading hypotheses include: "flushing out" toxic metabolic byproducts that build up in the brain during the day, consolidating and organizing memories, conserving energy, and regulating the body's broader metabolic processes.

Each of these theories has solid experimental evidence behind it, but none fully explains every observed feature of sleep on its own — for instance, why sleep duration varies so wildly between species, or why sleep deprivation in humans can, in extreme cases, kill faster than starvation.

In short: we know sleep is absolutely essential to life, but we still can't fully explain why it works this particular way, or why evolution never came up with anything cheaper.