The movement to a position earlier in the 24 hour sleep - wake cycle of a period of sleep or wake; for example, a shift of the sleep phase from 11 p.m. - 7 a.m. to 8 p.m. - 4 a.m. See Phase delay.

a shift earlier in time, for instance if someone starts going to bed earlier and waking up earlier.

shortening the sleep-wake cycle, as occurs when traveling from west to east. (199)