By the time the average person reaches his or her average life expectancy of around 80 years, they will have invested 28 years of their lives in sleep. It is remarkable that an activity of this scale is so taken for granted. Ironically, it is the defining characteristic of sleep ? perceptual disengagement from the environment ? that may provide the explanation for our disinclination to give our need for sleep its due attention. There is a natural tendency to invest effort in activities that provide conscious reward, and sleep risks being assigned a low priority compared with activities that occur during wakefulness. Importantly, these wakeful activities suffer where sleep is impaired.