Exogenous melatonin can be beneficial in different sleep disturbances including delayed sleep phase insomnia (Dahlitz et al. 1991, Regestein et al. 1995), melatonin- deficiency-related insomnia (especially in elderly) (Garfinkel 1995, Etzioni et al. 1995) and shift work sleep disorder (Folkard et al. 1993, Skene et al. 1996).