What is the evolutionary purpose of sleep? Doesn’t it seem like a huge concession to spend a third of your life inactive? Or is it an amazing strategic adaptation from the days before indoor lighting, when productivity went down once the sun set, and danger came in the form of fast and powerful predatory animals?
Jerome Siegel, a UCLA neuroscientist, suggests the latter. He says that for our ancestors, staying awake and mobile during the evening hours would actually have been evolutionarily maladaptive. We are not the brown bat, who can avoid predators by sleeping 20 hours a day and still manage to hunt blind at night via sonar.
It's been argued that the way our bodies recharge during our evening REM cycles—not just regaining energy but rebalancing the immune system and performing brain maintenance—is just another example of evolution piggybacking on an existing adaptation and making the most of it. We aren't equipped for echolocation, so this is what we got instead.
Of course, there is no way to know if Siegel’s theory is correct, but it’s an interesting idea. What’s not in question is that something like 80% of adults are sleep deprived. And a lack of sleep is unquestionably maladaptive.
Sleep deprivation increases your chances of cancer, obesity, heart disease, Alzheimer’s disease, dementia, high blood pressure, stress and depression. If that weren’t enough, we now know it compromises your ability to pick up and store new information—to learn, in other words.
It appears that your brain assimilates and consolidates semantic memory—facts, figures and bits of specific data—during the first stage of deep sleep. This means that if you’re studying for a test on the periodic table, it's not in your best interest to stay up cramming. You're just putting off one of the most key parts of the process.
During the last stage of sleep, which happens right before you wake up in the morning, your brain builds out episodic memory of physical activities, like learning dance steps or banjo licks, or any other form of performance-based learning. If your goal is to master the latest position from your yoga class, you’ll want to make sure you don’t wake up earlier than usual and cheat yourself.
So getting up extra early for marching band practice to run through drills is, by definition, self-defeating. And staying up late plays havoc with memorizing the state capitol of Pennsylvania (Harrisburg).
In his book How we Learn, Benedict Carey makes the point that ‘sleep is learning’. And for maximum learning to take place, there's no way around needing those eight hours a night for those sleep cycles to play out. Based on the latest research, it appears that Carey is correct. At the very least, it’s something to sleep on.