The mood-improving effect of sleep deprivation (SD) in depression is even today still not fully understood. Despite the fact that mood and cognitive functions are lowered by prolonged sleep loss and despite convincing data that insomnia is a strong risk factor for subsequent depression,(1) acute SD for one night or even partial SD in the second half of the night improves mood in about 60% of depressed patients the day after.(2,3) In this respect, among alt types of antidepressant treatments, SD elicits the fastest results, faster even than electroconvulsive therapy. Many authors correlate the likelihood of responding to SD with clinical variables. A summary of predictors is listed in Table I.