Circadian rhythm monitoring. Computer display showing a patient's brainwaves monitored over 24 hours, during a sleep deprivation study. EEG (electroencephalogram) traces of brain activity are seen in white on the screen; in yellow, peaks of brainwave activity are seen over a day. The patient, deprived of sleep, has been monitored for alertness, rest-activity and cycles of sleep. These cycles are controlled by a circadian rhythm, a human biological clock. By researching these rhythms, the alertness of night workers can be increased and sleep disorders can be treated. Photographed at the Human Alertness Research Center, Institute for Circadian Physiology, USA.