Male patient asleep in a sleep research laboratory, during a circadian rhythm experiment. Electrodes are attached to his head to measure brainwaves (EEG), eye and facial movements. Through the sound-proof port (top left) heart rate (ECG) is monitored and blood is sampled for hormone analysis. The patient is cut off from all references to the time of day. In this way, his circadian rhythm, an internal biological clock which controls awake and sleep periods, is being "timed" or "phased". This clock also controls daily changes in body temperature and hormone secretion. Photographed at the Laboratory for Circadian and Sleep Disorder Medicine in the USA.