The first extensive longitudinal electroencephalographic sleep investigation of a patient with a 48-hour "manic-depressive cycle" is reported. On nights preceding "depressed" days, the patient's sleep showed significantly more early morning awakening and less REM activity compared to nights preceding "hypomanic" days. Changes in REM activity throughout the night coincided with alterations in mood occurring throughout the night. Despite the inverse correlation of REM activity to depressed mood, a brief REM deprivation protocol failed to alter the periodicity of the mood cycles. Since depressed mood can be related to particular sleep alterations, it appears that the all night sleep pattern should be viewed as a dynamic process reflecting mood changes throughout the night.