Two patients ask me this question, and they ask it from opposite directions. The first has slept six hours a night for twenty years and feels fine. He wants to know whether the warnings apply to him. The second sleeps nine hours, wakes up tired anyway, and wants to know what is wrong. Until recently the honest answer was that sleep research mostly measured how people felt, or how long they lived, without much in between. A study published in Nature this year measured somethin