I posted earlier today about a book about sleep in the "What have you read recently?" thread. I just finished reading "Why We Sleep", subtitled Unlocking the power of sleep and dreams, by Matthew Walker, a sleep researcher at UC Berkeley. I highly recommend the book.
It's an account of what has been discovered in recent decades about sleep, not just in humans but in all animals, even insects. The difference between NREM (Non-Rapid Eye Movement) sleep and REM sleep, and the manner in which they change during during the course of a night's sleep, is absolutely fascinating, as is what is happening inside our brains during each type of sleep.
There are numerous intriguing factoids presented in the book. For example, the amount of sleep each species typically gets varies enormously, even within the same animal family, and it's not yet known why. The mammal thought to sleep the most is the brown bat, which averages 19 hours/day. Elephants need only 4 hours of sleep, whereas tigers and lions each sleep for 15 hours daily. The two halves of the brains of dolphins can de-couple, with one side sleeping while the other remains awake.
He also talks about the effects of different sleeping pills, as well as caffeine and alcohol. The biggest takeaway is the importance of getting at least 7 hours of sleep daily, and the effects of insufficient sleep.