The Science of Sleep, by developmental molecular biologist Dr. John Medina, gives you the facts-and the prescription to sleep well-in his signature engaging style. Adapted from Medina's Brain Rules for Aging Well: 10 Principles for Staying Vital, Happy, and Sharp, in this chapter you'll learn how the sleep cycle is born of a constant tension between hormones and brain regions vying to keep you awake, and hormones and brain regions trying to make you go to sleep. This is called opponent-process theory. Sleep, scientists are finding, doesn't have as much to do with energy restoration as it does with processing memories and flushing out toxins in the brain. As you grow older, your sleep cycle becomes more fragmented, particularly the part of the cycle during which toxins are flushed out of the brain. Accruing good sleep habits by middle age (a stable sleep routine; no caffeine, alcohol, or nicotine six hours prior to going to sleep) is the best way to avoid sleep-related cognitive decline in old age. You may already be experiencing the sometimes-unpleasant effects of the aging process. Or you may be deeply concerned about your loved ones who are. Either way, The Science of Sleep is for you.