This is the first book you will read where the author explicitly encourages the act of falling asleep while reading it. In it’s introduction, the author said he would take that as a compliment. “Why we sleep” is a very detailed account of the different functions of sleep and the investigations that revealed them both in humans and across many other species.

17 things you probably didn’t know, about why we sleep.

1) Birds can get non-REM sleep one hemisphere at a time and remain flying.

While humans cannot sleep one hemisphere at a time, in unfamiliar circumstances, let’s say a hotel, one hemisphere sleeps more lightly than the other. After a couple days at the hotel, sleep returns to normal. I wonder if the lighter hemisphere has any relation to handedness.
2) Sleep deprivation is very very bad.
Sleep deprivation is associated with psychological mood disorders, heart attacks & stroke, lower cognitive ability, stunted cognitive/emotional development in children, compromised immune systems, obesity and increased food cravings. There are people who’ve even died from sleep deprivation. In the later stages of death by sleep deprivation the immune system really isn’t functioning at all.
3) Cetaceans are the only mammals (and possibly the only known vertebrates) that don’t get REM sleep.
This is true at least after they’re born. REM sleep requires both hemispheres of the brain, and activates large sections of the brain. In order to keep us (and animals) from acting out our dreams, REM sleep requires that the body be temporarily paralyzed. Migratory birds cannot enter REM sleep while they’re flying (they’ll try to catch up after they land), but cetaceans need to constantly swim to the surface to breathe air, so they have to go without it.
4) We alternate between N-REM and REM sleep with an emphasis on N-REM early in the evening and REM in the morning before waking up.
Waking up early often interferes with the REM-heavy section of the sleep cycle, and can lead to negative developmental effects in children. Early school start times are directly correlated to increased traffic accidents near high schools.
5) N-REM sleep helps master motor skills, especially with things practiced at night before bed.
If for example you practice the piano and are having difficulty with one part, there’s a section of time (near 4:30-5am) when your brain replays that event and massages the parts where you were failing, so that when you wake up you improve. As it turns out perfect practice does not make perfect. Perfect practice plus sleep does.
6) REM sleep has the capacity for solving problems that we had while awake.
People given problems that require creativity to solve did vastly better if they were asked after being immediately woken (and still groggy) from REM sleep, versus during the day. When asked their reasoning, the people woke from REM reasoned a completely different way.
7) REM sleep replays emotionally troubling memories and stripping out the emotion, thus preventing PTSD.
There’s some evidence that soldiers with PTSD get recurrent nightmares, which prevent the normal REM process from stripping away that emotion. As a result, on the following night the REM sleep attempts to go over the same memories again and strip out the emotion, only leading to another nightmare. Drugs that suppress nightmares and enable proper functioning of REM sleep have had profound effects on people with PTSD.
8) REM sleep is where we extrapolate rules/concepts from examples.
Studies were done of students learning languages, and the part grammatical rules were extrapolated from examples did now function properly if REM sleep was interrupted. This can also be suppressed by alcohol, so watch out.
9) Alcoholic withdrawal hallucinations come for a massive pressure of missed REM sleep.
Alcohol is an extremely potent suppressor of REM sleep. In the study mentioned above, the control group of students learning the language went straight to sleep, while the first experimental group had the equivalent of three shots right before bed on the first night, and the second experimental group had the equivalent of three shots on the third night. At the end of a week they were tested. Unsurprisingly, the people who got drunk right before bed on the night they studied did 50% worse than those who never drank. Surprisingly, the people who got drunk on the third night did 40% worse than the control group. I guess if you must drink at all, try a bourbon for breakfast.
10) Fetuses will spend as much as 12 hours a day in REM sleep in the weeks approaching birth.
REM sleep is also where new neuron connections are created, and N-REM sleep is where they are pruned. Infants and children spend more time in REM sleep compared to N-REM sleep. The balance shifts the other way as they mature.
11) Even worms, insects and jellyfish sleep.
In order to do this, scientists had do develop a fairly rigorous behavioural definition of sleep. It goes something like:

a) The animal assumes a special posture with limited movement and stays there for an extended period of time.
b) The animal returns to that posture roughly the same time every day (or according to some fixed schedule)
c) When disturbed so that the animal cannot stay in that sleep posture, the animal shows decreased activity the next day, and spends more time in the sleep posture the following day. In other words the jellyfish was groggy and slept in late because you kept it up all night, you monster.

12) Interrupting REM sleep in baby rats pauses cognitive development, ultimately stunting it.
As it turns out we only have a limited window for cognitive development where new neurons and connections are taking place. If you prevent a fetus from getting REM sleep for two weeks by stimulating it with noise, the brain will not develop during that time. Afterwards it will resume its development, but no faster than before, meaning it will never catch up. There is some speculation that this is how fetal alcohol syndrome works to mentally retard children.
13) Sleep preserves memories selectively.
Yes, you have a selective memory, but it’s not your fault! You were asleep! Take this excuse and run with it.
14) Children’s circadian rhythm is naturally earlier than adults and adolescent rhythm is naturally later.
As it turns out teenagers are not lazy for naturally feeling an urge to sleep in. I feel some portion of my adolescence has been justified! On a side note, there is some speculation that this is an evolutionary drive for individuation on the part of the growing child, to encourage him to spend more time away from his parents. Having an offset sleep schedule is an easy way to accomplish that.
15) Morning larks and night owls are likely genetic.
Take that, Dad!
16) People dreaming about a problem are more likely to solve it.
It was said that Thomas Edison had a special chair that he would sleep in, with a metal pan under it. He would hold a metal ball in his hand as he fell asleep. Then, as he started to enter REM and his hand relaxed, the ball would fall from his hand and hit the pan with such a clang that he jumped out of sleep, with dreams and inspirations intact.
17) Risk of medical malpractice is directly correlated with how much sleep the staff got.
The next time I talk to a Surgeon, I’m looking for bags under his eyes, and I will ask how much sleep he got.

The book covers a lot more than what I just described. I definitely recommend reading it, maybe right before bed.