SUMMARYThe aim of this study was to explore the mediating role of sleep characteristics in the relationship between morningness-eveningness and three different aspects of daytime functioning: daytime sleepiness, depressive mood and substance use in university students. A multiple mediator model was proposed with sleep debt, poor sleep quality and bedtime delay at weekends as parallel mediators in these relationships. We analysed the data of 1052 university students aged 18-25 years who completed a modified version of the School Sleep Habits Survey, which included questions on sleep and the Composite Scale of Morningness, Sleepiness Scale, Depressive Mood Scale and Substance Use Scale. Students with more pronounced eveningness reported greater daytime sleepiness, greater depressive mood and more frequent substance use, as well as greater sleep debt, poorer sleep quality and greater bedtime delay at weekends. Mediation analyses indicated that morningnesseveningness affected daytime sleepiness and substance use both directly and indirectly through all proposed sleep-related mediators. However, the effect of morningness-eveningness on depressive mood was entirely indirect and was accounted for more by poor sleep than by sleep debt or bedtime irregularity. In conclusion, there are multiple possible mechanisms through which morningness-eveningness affects daytime functioning in university students, and sleep characteristics are a significant mechanism. Sleep debt, poor sleep quality and bedtime irregularity can, to a significant extent, explain the feeling of daytime sleepiness and greater substance use in students with eveningness preferences. However, more depressed mood in the evening-orientated students is primarily a consequence of their poor sleep quality.
The results indicate the need for the continuation of efforts to establish effective primary preventive programmes for OSDs at national and EU levels.
Educational leaflets can be an effective first step in educating younger high school students about healthy sleep, with the method being more effective in female adolescents.
School system in which classes are scheduled 1 week in the morning and the other in the afternoon, and in which students rotate schedule every week, fosters sleep irregularity. In this study, we examined morningness-eveningness of adolescents who were involved in such schedule of school time and explored relationship between their circadian preferences and sleep characteristics. A large sample of 2287 students between the ages 11 and 18 years (52% girls) from 24 schools in Croatia was studied. The School Sleep Habits Survey was modified to enable differentiation of sleep patterns between the two school schedules and weekends. Two measures of ME were used: the Morningness-Eveningness Scale for Children (MESC) and mid-sleep time on weekends (MSFsc). Both measures showed a shift to eveningness starting between the ages 12 and 13 (MESC), or 13 and 14 (MSFsc). However, MESC demonstrated a plateau in the shift in older adolescent whereas MSFsc indicated further progress of phase delay. Significant differences in sleep timing and duration were found between three chronotype groups (Morning, Intermediate, and Evening). Generally, Evening types went to bed and woke up the latest in all situations. Their sleep duration was the shortest on school week with morning schedule. On weekends Morning types slept shorter than other two chronotype groups. On school week with afternoon schedule all chronotype groups slept close to the recommended 9 h. All three chronotype groups delayed their bedtimes and wake-up times, and extended their sleep in situations with fewer constraints on sleep timing (i.e. afternoon school schedule, and weekends versus morning school schedule). Expectedly, the evening types showed the greatest sleep irregularity. The findings of this study suggest that the Croatian school system fosters sleep irregularity, but provides more opportunity for fulfilling sleep need of all chronotype groups of adolescents. Age effects on morningness-eveningness observed in Croatian adolescent do not seem to be different from those observed in adolescents from other countries involved in a regular morning school schedule. Further studies are necessary to explore differences in the trend of shift towards eveningness found between the two measures of morningness-eveningness in this, as well as in other studies.
It is generally accepted that two basic physiological processes regulate sleepiness: homeostatic and circadian rhythmic processes. Recent research has proposed the third component regulating sleepiness, that is, the wake drive or the arousal system. The role of the arousal system in regulating sleepiness has partly been addressed by the studies of the pathophysiology of insomnia, which is often described as a disorder of hyperarousal. Experimental and correlational studies on the relation between sleepiness and arousal in good sleepers have generally indicated that both physiological and cognitive arousal are related to the standard measures of sleepiness. Taking into account the role of the arousal system in regulating sleepiness widens the possibilities for the management of sleep disorders and could also help in solving the problem of excessive sleepiness at work and the wheel. KEY WORDS: circadian rhythms, insomnia, shift work, sleep homeostasis, wake driveBakotić M, and Radošević-Vidaček B. WAKE DRIVE IN SLEEPINESS REGULATION Arh Hig Rada Tokiskol 2012;63 Supplement 1: 23-34 Sleep research and sleep medicine often defi ne sleepiness as a state refl ecting physiological sleep need (1). Other terms used in addition to sleepiness are sleep propensity and sleep tendency. Sleepiness is measured by how long it takes to fall asleep, with tests such as the Multiple Sleep Latency Test (MSLT) (2) and the Maintenance of Wakefulness Test (MWT) (3). Sleepiness is also measured by behaviour such as eye closing, yawning, specifi c head movement (bobbing), facial expressions, and performance in various psychomotor and cognitive tasks (4). In the subjective domain, measurements include various single-or multi-facetted sleepiness rating scales (5, 6) and scales assessing the tendency to fall asleep in various situations (7).Although sleepiness is a normal physiological state related to sleep need, in certain cases it can be a symptom of a serious disease or condition. In such cases the term pathological or excessive sleepiness is used. The American Academy of Sleep Medicine defi nes excessive sleepiness as sleepiness that occurs in situations when a person is expected to be awake and alert (8), which interferes with person's daily functioning such as work or school performance, cognitive functioning, mood, and social interaction.Empirical data indicate that sleepiness signifi cantly contributes to vehicle accidents and that mortality in these accidents is very high, especially in persons younger than 25 years (9-12). Furthermore, sleepiness at the workplace is associated with lower performance and greater risk of occupational accidents and injuries, especially in industrial operations and transportation services (10,(13)(14)(15). Shift workers run the greatest risk of sleepiness at the workplace because their working time often coincides with the time of the day when humans are biologically programmed to sleep. 24On the other hand, shift workers often sleep at the time when circadian factors promote wakefulness, which c...
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