Indeed, sleep is a highly complex state arising from an interaction between multiple brain regions, neurotransmitter pathways and hormones, none of which are exclusive to the generation of sleep. The sleep–wake cycle is the most familiar 24-hour cycle, but involves more than the SCN. If this entrainment pathway is disrupted at any point, then the biological clocks in different tissues can become uncoupled, resulting in a state of internal desynchrony of the 24-hour oscillator network (Foster et al. The SCN in turn co-ordinates the activity of the entire circadian network and most 24-hour behavioural, metabolic and physiological rhythms including alertness and performance, hormones, core body temperature, metabolism and the different rhythms in key organs such as the lungs, liver and heart (Hilton et al. The SCN is entrained to the environmental light–dark cycle by specialized photoreceptors within the eye (Foster and Hankins 2007 Lockley and Foster 2012). This is achieved via a master circadian (24-hour) pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Every cell in the body has cellular oscillators, and these need to function in synchrony with each other and with the environmental day. Like most life on earth, our physiology and behaviour shows a 24-hour rhythm. The circadian timing system, sleep and adolescence: learning, mood and health These assumptions reflect societies’ prejudice in favour of early risers in adulthood, exemplified by the proverb: Educators tend to think that adolescents learn best in the morning and if they simply went to sleep earlier, it would improve their concentration. The impact of early school times on adolescents is not understood by most educators: a common belief is that adolescents are tired, irritable and uncooperative because they choose to stay up too late, or are difficult to wake in the morning because they are lazy. This level of sleep loss causes impairment to physiological, metabolic and psychological health in adolescents while they are undergoing other major physical and neurological changes (Hansen et al. Failure to adjust education timetables to this biological change leads to systematic, chronic and unrecoverable sleep loss. Thus, a 07:00 alarm call for older adolescents is the equivalent of a 04:30 start for a teacher in their 50s. At its peak the combination of these two biological changes leads to a loss of two to three hours sleep every school day. During adolescence biological changes dictate both a sleep duration of nine hours and later wake and sleep times, a phenomenon found in other mammals (Hagenauer et al. In contrast, in late adolescence the conflict between social time and biological time is greater than at any point in our lives. When social time and biological time are more closely aligned, as in the early years of education, this distinction is not critical. It is no surprise given the relative novelty of mechanical clocks in evolutionary timescales that our ability to function optimally, including in learning, varies with biological time rather than conventional social times. In contrast, biological time is measured in developmental changes in the body, and over the day by our internal biological clock. For the educator, student development is defined by age and the daily timetable by social conventions that vary between countries, regions and even individual schools. Time is a fundamental variable in human biology and in education, yet currently the two disciplines measure time in different ways.
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