Why do we sleep? If sleep doesn’t serve some essential function, then it is evolution’s biggest mistake, according to one evolutionary scientist quoted in Matthew Walker’s book Why We Sleep: Unlocking the Power of Sleep and Dreams, which explains what sleep seems to do for us, what sleep deprivation does to us, and why we should all be getting more sleep and encouraging our kids and our employees to do the same.

Walker, a sleep researcher and Professor of Neuroscience and Psychology at Cal-Berkeley, begins by delving into what we know about the history of sleep in humans, and how sleep itself is structured. Humans were, for most of our history as a species, biphasic sleepers – we slept twice in each 24 hour period. We retain vestiges of this practice, which only ended in the 19th century in the developed world with the Industrial Revolution, in our Circadian rhythms, which still give us that post-prandial ‘slump’ that led to customs like the siesta. (It had never occurred to me that the word “circadian” itself came from the Latin words for “almost a day,” because that rhythm in our bodies isn’t quite 24 hours long.)

Sleep is, itself, two different processes that occur sequentially, alternating through a night of full sleep. Most people are familiar with REM sleep, referring to the rapid eye movements visible to an observer standing not at all creepily over you while you slumber. The remaining periods of sleep are, creatively, called nREM or non-REM sleep, and themselves comprise three different sub stages. Both phases of sleep are important; REM sleep is when dreaming occurs, which itself seems to serve the purposes of helping the brain process various events and the associated emotions from the previous day(s), as well as allowing the brain to form connections between seemingly unrelated memories or facts that can seem like bursts of creativity the next day. Your body becomes mostly paralyzed during REM sleep, or else you’d start moving around while you dream, perhaps kicking, flailing, or even acting out events in your dreams – which can happen in people with certain rare sleep disorders. N-REM sleep allows the body to repair itself, helps cement new information into memories in the brain’s storage, boosts the immune system, and contributes to feelings of wakefulness in the next day. The part of N-REM sleep that accomplishes the most, called deep or N3 sleep, decreases as you age, which is why older people may find it hard to sleep longer during the night and then feel less refreshed the next morning.

The bulk of Why We Sleep, however, is a giant warning call to the world about the hazards of short- and long-term sleep deprivation, which Walker never clearly defines but seems to think of as sleeping for a period of less than six hours. (He calls bullshit on people, like our current President and I believe his predecessor too, who claim they can function well on just four or five hours of sleep a night.) Sleep deprivation affects cognition and memory, and long-term deprivation contributes to cancer, diabetes, mental illnesses, Alzheimer’s, and more. Rats deprived of sleep for several days eventually die of infections from bacteria that would normally live harmlessly in the rats’ intestinal tracts.

We don’t sleep enough any more as a society, and there are real costs to this. Drowsy driving kills more people annually than drunk driving, and if you think you’ve never done this, you’re probably wrong: People suffering from insufficient sleep can fall into “micro-sleeps” that are enough to cause a fatal accident if one occurs while you’re at the wheel. Sleep deprivation in adolescents seems to lead to increased risks of various mental illnesses that tend to first manifest at that age, while also contributing to behavioral problems and reducing the brain’s ability to retain new information. Walker even ends the book with arguments that corporations should encourage better sleep hygiene as a productivity tool and a way to reduce health care costs, and that high schools should move their school days back to accommodate the naturally later sleep cycles of teenagers, whose circadian rhythms operate somewhat later than those of preteens or adults.

One major culprit in our national sleep deficit — which, by the way, isn’t one you can pay; you can’t ‘catch up’ on lost sleep — is artificial light, especially blue light, which is especially prevalent in LED light sources like the one in this iPad on which I’m typing and the phone on which you’re probably reading this post. Blue light sources are everywhere, including the LED bulbs the environmentally responsible among us are now using in our house to replace inefficient incandescent bulbs or mercury-laden CFLs. Blue light confuses the body’s natural melatonin cycle, which is distinct from the circadian rhythm, and delays the normal release of melatonin in the evenings, which thus further delays the onset of sleep.

Sleep confers enormous benefits on those who choose to get enough of it, benefits that, if more people knew and understand them, should encourage better sleep hygiene in people who at least have the discretion to sleep more. Sleep helps cement new information in your memory; if you learn new information, such as vocabulary in a foreign language, and then nap afterwards, you’re significantly more likely to retain what you learned afterwards. Sleep also provides the body with time to repair some types of cell damage and to recover from muscle fatigue – so, yes, ballplayers getting more sleep might be less prone to injuries related to fatigue, although sleep can’t repair a frayed labrum or tearing UCL.

Walker says he gives himself a non-negotiable eight-hour sleep window every night. I am not sure how he can reconcile that with, say, his trans-Atlantic travel, but he does point out that changing time zones can wreak havoc on our sleep cycles. He suggests avoiding alcohol or caffeine within eight hours of bedtime — so, yes, he even says if you want that pint of beer, have it with breakfast — and offers numerous suggestions for preparing the body for sleep as you approach bedtime, including turning off LED light sources, using blue light filters on devices if you just can’t put them down, and even using blackout shades for total darkness into the morning.

There are some chapters in the middle of Why We Sleep that would stand well on their own, even if they’re not necessarily as relevant to most readers as the rest of it. The chapter on sleep disorders, including narcolepsy and fatal familial insomnia (about as awful a way to die as I could imagine), is fascinating in its own right. Walker also delivers a damning rant on sleeping pills, which produce unconsciousness but not actual sleep, not in a way that will help the body perform the essential functions of sleep. He does say melatonin may help some people, although I think he believes its placebo effect is more reliable, and he questions whether over the counter melatonin supplements deliver as much of the hormone as they claim they do.

Why We Sleep was both illuminating and life-altering in the most literal sense: Since reading it, I’ve set Night Shift modes on my devices, set alarms to remind me to get to bed eight hours before the morning alarm, stopped trying to make myself warmer at night (cold prepares the body for sleep, and you sleep best in temperatures around 57 degrees), and so on. I had already been in the habit of pulling over to nap if I became drowsy on a long drive, but now I build more time into drives to accommodate that, and to give myself more time to wake up afterwards – Walker suggests 20 minutes are required for full cognitive function after even a brief nap. Hearing the health benefits of sleeping more and risks of insufficient sleep, including higher rates of heart disease, cancer, and Alzheimer’s, was more than enough to scare me straight.

Next up: I’m halfway through Brian Clegg’s A Brief History of Infinity: The Quest to Think the Unthinkable.