A temporal discrepancy between the endogenous sleep-wake cycle and the daily structure of the surrounding social network are characteristic for chronobiological sleep disturbances. Activity rhythms that are in abnormal relation to the environment are more frequent than commonly assumed. They can arise either from external causes (such as shift-work or jet-lag) or as a result of internal changes promoting abnormal sleep behaviour. Structuring daily activities by paying attention to natural daylight (dawn and dusk) and to the social routine strengthen the synchronizing effect of external timekeepers necessary for the concordance between inner and outer rhythmic phenomena. Treatment of chronobiological sleep/wake-cycle disturbances require correct diagnosis and modification of their causes, particularly changes in habits consolidating such disturbances. Early recognition of a chronobiological sleep disorder can reduce the risk of misuse of sleeping pills, caffeine and nicotine. Recently developed treatment approaches such as bright light, the pineal hormone melatonin and vitamin B12 have provided promising results.