SLEEP AND THE DEPRIVATION OF SLEEP ARE BECOM- ING IMPORTANT ISSUES IN EVERYDAY LIFE. SLEEP DE- PRIVATION HAS CONSEQUENCES FOR PUBLIC safety

SLEEP AND THE DEPRIVATION OF SLEEP ARE BECOMING IMPORTANT ISSUES IN EVERYDAY LIFE. SLEEP DEPRIVATION HAS CONSEQUENCES FOR PUBLIC safety and health. Some obvious problems concern attention, vigilance, and fatigue,1 but less obvious ones are cardiovascular problems and obesity.2 Sleep deprivation is known to cause molecular, physiological, and behavioral changes. It changes the expression of immediate early genes, transcription factors, and genes related to energy metabolism in the brain3 and influences leptin and grehlin levels causing increased hunger and high calorie food cravings.4 Also the functioning of the brain, measured by magnetic resonance imaging5 or electroencephalogram (EEG) recordings6, is changed after sleep deprivation. Sleep wake cycles are regulated by the interplay of a homeostatic mechanism, which regulates sleep depth, and the circadian pacemaker of the suprachiasmatic nucleus (SCN), which regulates timing of sleep.7 Deprivation of sleep has major effects on the homeostatic sleep regulatory mechanisms but is assumed to have minor or no effects on the circadian pacemaker. The effects on sleep homeostasis explain why animals, ranging from insects8 to mammals9 sleep more deeply after sleep deprivation, and sleep occurs outside the normal rest period after sleep deprivation.10,11 In humans, sleepiness is increased during the day following sleep deprivation.12 The timing of sleep outside the rest phase is surprising given the assumption that the circadian pacemaker is largely unaffected by sleep deprivation. The findings can be explained in 2 ways; either the homeostatic mechanism overrules the circadian clock, or the functioning of the circadian pacemaker is altered by sleep deprivation. We investigated the latter option by recording SCN electrical activity patterns during and after a 6-h sleep deprivation period, and we observed a long term suppression of the electrical activity rhythm of the SCN.