Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann
A shift of an episode of sleep or wake to a later time of the 24 hour sleep-wake cycle. It is the exact opposite of phase advance. These terms differ from common concepts of change in clock time: to effect a phase delay, the clock is moved ahead or advanced. In contrast, to effect a phase advance, the clock moves backward. See phase advance.