The antidepressant mechanism of action of sleep deprivation is not known as yet; several hypotheses are presented and discussed in various review articles (Wiegand 1995; Kasper & Möller 1996; Wirz-Justice & van den Hoofdakker 1999; Gillin et al. 2001; Ringel & Szuba 2001; Giedke & Schwärzler 2002). At present, the following general hypotheses are most discussed: