My mother used to say, “The two hours of sleep I get before midnight are the best two hours of the night.” I wondered about that until I read comments by Matthew P. Walker, PhD, director of the Sleep and Neuroimaging Lab at Beth Israel Deaconess Medical Center. The earlier in the night, the greater the propensity for deep non-REM (rapid eye movement) sleep, and the later in the morning, the greater the propensity for REM sleep. Therefore, someone who sleeps from 9p.m. to 5a.m. (8 hours total) will have a different overall composition of sleep—biased towards more non-REM—than someone who sleeps from 3a.m. to 11a.m. (also 8 hours total), who is likely to experience more REM. Going to bed too late, then, will deprive you of some of the restorative functions that non-REM sleep normally provides. More tomorrow.