For most of the twentieth century, sleep was (0) REGARDED by scientists as a kind of biological : a passive state during which the brain powered down and little of consequence occurred. That picture has been quietly over the past four decades. Neuroscientists now view sleep as an intensely active period, one during which the brain the day's experiences, clears metabolic waste, and lays down the long-term memories that wakeful hours could never process on their own. The implications are far-reaching. Chronic sleep deprivation, once off as a minor inconvenience, is now to a striking range of cognitive, emotional and metabolic harms. Night-shift workers, sleepless parents and teenagers into ever later bedtimes all pay a biological price that the older science could not have detected. Whether public policy will up with the new understanding remains an open question. School start times, working hours and hospital rotas are only slowly beginning to the reality that sleep is not lost time but the very foundation on which waking competence rests.