National Geographic : 2010 May
• treatable through their tool kit, usually anxiety or depression. By extension, cognitive behavioral therapy asks the sleeper to think about what he or she is doing wrong, not what is wrong with his or her body. Winkelman wishes that the two aspects of sleep---the physical and the mental--- were more o en considered jointly. "Sleep is extraordinarily complicated," he says. "Why would we think that there couldn't be some- thing in the wiring that goes awry too?" ' , perhaps it's because we've forgotten how. In premodern times people slept differently, going to bed at sunset and rising with the dawn. In winter months, with so long to rest, our ancestors may have broken sleep up into chunks. In developing countries people still o en sleep this way. ey bed down in groups and get up from time to time during the night. Some sleep outside, where it is cooler and the e ect of sunlight on our circadian rhythm is more direct. In 2002, Carol Worthman and Melissa Melby of Emory University published a comparative survey of how people sleep in a variety of cultures. ey found that among for- aging groups such as the !Kung and Efe, "the boundaries of sleep and waking are very uid." ere is no xed bedtime, and no one tells any- one else to go to sleep. Sleepers get up when a conversation or musical performance intrudes on their rest and intrigues them. ey might join in, then nod o again. No one in developed nations sleeps this way today, at least not on purpose. We go to bed near a xed time, sleep alone or with our partner, on so cushions covered with sheets and blankets. We sleep on average about an hour and a half less a night than we did just a century ago. Some of our epidemic of insomnia or sleeplessness is probably just our refusal to pay attention to our biology. e natural sleep rhythms of teenagers would call for a late morning wake-up---but there they are, starting high school at 8 a.m. e night shi worker sleeping in the morning is ghting ancient rhythms in his or her body that order him or her awake to hunt or forage when the sky is ooded with light. Yet he or she has no choice. We ght these forces at our peril. In February 2009 a commuter jet en route from Newark to Bu alo crashed, killing all 49 aboard and one on the ground. e copilot, and probably the pilot, had only sporadic amounts of sleep the day leading up to the crash, leading the National Transportation Safety Board to conclude that their performance "was likely impaired because of fatigue." is sort of news enrages Harvard's Charles Czeisler. He notes that going without sleep for 24 hours or getting only ve hours of Cells in the retina use light to set the brain's circadian clock. Studies at Brigham and Women's Hospital in Boston suggest blue light resets the clock most e ciently---holding promise for jet-lagged travelers and drowsy night shi workers.