The results of studies throughout the world showed significant changes in sleep characteristics in adolescents: as they get older, they manifest a greater tendency towards later bedtimes and later wake-up times. Those sleep characteristics are a consequence of changes in biological mechanisms of sleep-wakefulness regulation – a shift in the phase of endogenous circadian rhythms towards later hours and a slower accumulation of sleep pressure during the day. Various external factors also influence sleep characteristics of adolescents, among which school schedule is one of the most important. In many countries adolescents attend classes which start in the morning. Therefore, throughout the school week their wake-up time is determined by early school start time. For that reason, they sleep less than needed during the school week. To compensate for sleep loss and achieve necessary sleep duration, adolescents prolong their sleep during weekends and other days off. It has been shown that insufficient sleep during school days has a negative impact on daily functioning, academic performance and behaviour. The studies examining Croatian adolescents who attend school one week in the morning and the other in the afternoon indicated some advantages of double-shift school scheduling regarding sleep duration and daily functioning of adolescents.

wakefulness, sleep, biological mechanisms, circadian rhythms, homeostatic pressure