This article makes an enlightening point: SAD could be a form of jet lag, a disturbance in the circadian rhythm, the 24-hour pattern that normally aligns the sleep-wake cycle with all the other bodily rhythms. Dr. Lewy suggests that with the delayed dawn and shorter days of fall and winter, the rhythms of people afflicted with SAD drift out of phase with the sleep-wake cycle, as if they had traveled across many time zones.