getting one extra hour of sleep each night would do more for a person’s daily happiness than getting a $60,000 raise. Nevertheless, the average adult sleeps only 6.9 hours during the week, and 7.9 on the weekend—20 percent less than in 1900. Although people adjust to feeling sleepy, sleep deprivation impairs memory, weakens the immune system, slows metabolism, and might, some studies suggest, foster weight gain.