I read this book (by a sleep scientist called Matthew Walker) because I knew that it would tell me to sleep more, and I hoped it would cite enough scary statistics that I'd be likely to actually follow through. Well, it worked - I'm keeping a copy on my bedside table for the foreseeable future, just as a reminder. In addition to the exhortations to get more sleep, it contains a variety of other interesting and important facts about sleep.

What is sleep?

• Human sleep consists of cycles lasting about 1.5 hours, each of which contains first a period of NREM (Non-Rapid Eye Movement) sleep, then a period of REM sleep. In brain scans, the former consists of slow, deep brain waves, while the latter shows the same frenetic activity as an awake brain. As the night goes on, cycles feature a higher proportion of REM sleep. This means that if you cut your sleep short by 25%, you're actually missing out on somewhere between 60% and 90% of REM sleep. 
• REM sleep is when the majority of dreams happen. While it's uncommon for dreams to replay events from our everyday lives, they do often reflect our emotional preoccupations. To prevent ourselves from flailing around during dreams, we enter a state of sleep paralysis, where our brains are unable to control our voluntary muscles. Eyes are an exception - hence the name REM. It's definitely not true that REM is the only valuable type of sleep - in fact, immediately after sleep deprivation the brain prioritises catching up on NREM.
• The slow waves of NREM sleep are useful for transferring memories from one part of the brain to the other - in particular, from short- to long-term storage.
• Walker's theory is that NREM sleep is used to prune away unnecessary connections, while REM reinforces useful connections. He uses the analogy of a sculptor who alternates between carving away whole chunks of marble (NREM) and then adding fine detail on whatever's left (REM). From this perspective, it makes sense that REM sleep is concentrated in later cycles. However, it's unclear whether this is the scientific consensus.
• There are two systems controlling sleep and wakefulness. The circadian system follows the day/night cycle, making you tired in the evening and alert in the morning (the exact timings vary by person, making some people "night owls" and some "morning larks"). In addition, "sleep pressure" is controlled by adenosine, which builds up while you're awake and is cleared away during sleep. Caffeine works by temporarily blocking adenosine receptors, but doesn't prevent it from continuing to build up.

What's it good for?

• There's a very strong link between NREM sleep and memory. The formation of long-term memories suffers if we don't get enough sleep (even several days after the events we want to remember). This is true both for memories about facts and experiences and for "muscle memory" of actions like playing an instrument. When sleep-deprived, we also have worse short-term memory.
• REM sleep is important in emotional regulation and creativity. After sleep deprivation, the responses of the amygdala (responsible for strong emotions) can be amplified by over 60%, due to weakened links between it and the prefrontal cortex (responsible for "rational" decision-making). Dreams during REM sleep allow us to make unusual and creative connections between different topics - many great intellectuals report that their best ideas just "came to them" upon waking.
• Sleep deprivation massively reduces our ability to concentrate. In addition to slower reaction times, when tired we lapse into "micro-sleeps" during which we're totally unresponsive. Walker emphasises that tiredness is a far bigger cause of traffic accidents than drunk-driving, that drivers systematically underestimate how tired they are, and that drivers who micro-sleep often don't brake at all before collisions.
• In the long term, sleep deprivation increases the risk of Alzheimer's (since toxins are flushed from the brain during sleep), heart attacks (by provoking a stress response from the sympathetic nervous system and raising blood pressure) and cancer (by devastating the immune system). All of these seem to be very big effects - e.g. sleep-deprived patients are twice to three times as likely to suffer calcification of their coronary arteries.
• Note that most of the effects above are noticeable even after small amounts of sleep deprivation, like getting one or two hours less sleep for one or two nights. In fact, even the one-hour sleep reduction from Daylight Savings Time causes a spike in heart attacks.
• Sleep is also linked to many mental illnesses - e.g sleep deprivation triggers mania or depression in bipolar patients. Most mental illnesses disrupt sleep, which exacerbates their other negative effects.
• REM sleep promotes the formation of neural links in infants, who have far more neural connections than adults. It is also important for their language learning.
• Walker's broad answer to the question of what sleep is useful for: EVERYTHING. In addition to the above, sleep helps us overcome traumatic memories, reduces athletes' injury rates, makes us look more attractive, reduces food cravings, and so on and so on...

The evolution of sleep

• I guess it shouldn't be a surprise that sleep is so broadly useful: once it started, it makes sense that many metabolic processes would take advantage of it. And they've had a long time to do so: sleep is ancient, with all animal species demonstrating some form of sleep-like behaviour. 
• Even unicellular bacteria have active and passive phases corresponding to the planet's light/dark cycle.
• However, the length of sleep required varies wildly for different animals, from 4 hours for elephants to 19 for brown bats. 
• Only birds and mammals have proper REM sleep - it is a relatively recent adaptation. It also seems to be absent in aquatic mammals, whose two brain hemispheres sleep separately. 
• Humans seem to be naturally biphasic: modern hunter-gatherer tribes sleep for 7-8 hours at night, and then nap for 30-60 minutes in the afternoon. It's biologically natural to be sleepy after lunch. Biphasic sleep significantly decreases mortality from heart disease. 
• Walker hypothesises that descending from the trees to sleep on the ground allowed us to gain more REM sleep (particularly difficult in trees due to sleep paralysis), and therefore was important in boosting human cognitive development; also, that fire was vital in making ground-sleeping safer.

How to sleep better

• Alcohol is an extremely powerful suppressor of REM sleep. Since it stays in your system for hours, it's best not to drink in the evenings. 
• Light, especially blue light, signals your circadian system to wake up. Unfortunately LED screens provide a lot of blue light. Avoid using screens in the hours before bed, or at least phase out the blue light (e.g. using flux). 
• In addition to light, our bodies use decreasing temperatures as a signal to sleep. Lowering room temperature often helps with insomnia. Apparently your core temperature will also fall after a hot bath. 
• Caffeine has a half-life of 5 to 7 hours, so if you drink it in the afternoon, a significant amount will still be in your system at bedtime. 
• The circadian rhythm of a teenager is naturally a few hours later than that of an adult, so teens shouldn't be forced to get up too early. Unfortunately, schools aren't taking much notice of this. 
• Apparently sleeping pills cause lower-quality sleep and have severe long-term side-effects, so they should be avoided (with the exception of melatonin). 
• For serious sleep problems, Cognitive Behavioural Therapy for Insomnia (CBT-I) works fairly well and should be the first step.

As you can probably tell from the above, Walker is very much a cheerleader for sleep. This does bias him in some noticeable ways - e.g. his overt scorn towards coffee. He also blurs causation and correlation at some points throughout the book, so I'd be surprised if all of the deleterious effects mentioned above are as significant as he claims. But the overall picture is stark enough that I'm now very worried about the ongoing sleep loss epidemic.