The majority of studies in this field suffers from a methodological problem: there is no objective continuous polysomnographic measurement of sleep. The continuous absence of sleep during the sleep deprivation period is thus not documented. It is known from sleep deprivation studies in healthy probands that during prolonged sleep deprivation, short sleep episodes ("microsleep") occur frequently. Hemmeter et al. (1998) were the first to demonstrate that also in depressed patients undergoing sleep deprivation, microsleep occurs and tends to prevent the antidepressant effect. Data from a recently finished study of our group point into the same direction (partly published in Wiegand et al. 2002).