Disturbed sleep is a central feature of depression. Sleep may be construed in terms of sleep continuity ϵ, sleep architecture, and sleep micro-architecture, as well as various subtypes of these patterns. Investigation of the role of circadian rhythms in depression has generally supported a phase advance of the circadian rhythm. An alternate approach to studying brain activity during sleep is through the use of neuro-imaging, which has produced data that help to clarify the earlier rapid eye movement (REM) and non-rapid eye movement (NREM) findings. A number of investigators have examined the clinical correlates of sleep disturbance in depression. Clearly, sleep disturbance and depression are intricately intertwined, although the direction of causation and potential interactions are less certain. There is a long history of research investigating the relationship between sleep disturbance and depression. Sleep abnormalities include disturbances in sleep continuity, sleep architecture, quantitative EEG, circadian rhythms, and CNS arousal.