Several authors have verified that adolescents' sleep patterns exhibit some special features that distinguish them from other age cohorts. In summary, they show a sleep phase delay compared with patterns in younger children, a more irregular sleep schedule across the week, and a shorter sleep length on weekdays (Anders, Carskadon, & Dement, 1980; Rugg-Gunn, Hackett, Appleton, & Eastoe, 1984; Strauch & Meier, 1988). Despite the reduced amount of sleep on weekdays, the need for sleep does not decrease and adolescents tend to extend sleep on weekends and holidays (Levy, Gray-Donald, Leech, Zvagulis, & Pless, 1986; Strauch & Meier, 1988; Andrade, Benedito-Silva, Domenice, Arnhold, & Menna-Barreto, 1993; Szymczak, Jasinska, Pawlak, & Zwierzkowska, 1993). They show a high incidence of daytime sleepiness complaints, trouble in falling asleep at night, and difficulty in waking up in the morning (Bearpark, 1986; Andrade, Benedito-Silva, Domenice, Arnhold, & Menna-Barreto, 1993; Saarenpää-Heikkilä, Rintahaka, Laippala, & Koivikko, 1995).

Various factors may relate to adolescents' sleep patterns, probably many of them interacting with one another. Factors include an increase of school and social commitments (Allen, 1992; Manber et al., 1995); psychological variables such as depression, anxiety, and worry (Price, Coates, Thoresen, & Grinstead 1978; Bearpark, 1986; Carskadon, Seifer, Davis, & Acebo, 1991); and ontogenetic changes in the mechanisms of the biological clock.