Periodic movements during sleep (PMS) are discussed in relation to several hypotheses concerning their etiology and their possible nosographic classification. In particular the role of central and peripheral events able to determine PMS is emphasized. The possible existence of space-intensity-time (SIT) patterns responsible for the vigilance profiles and the state of activity of central and peripheral effectors is also discussed. Finally the authors examine the modulating role of GABA on the circadian system and a possible metabolic instability of the GABAergic neurons responsible for PMS. This hypothesis is confirmed by the usefulness of clonazepam to treat such disorders.