Sleep is a universal behaviour that has been demonstrated in every animal species studied, from insects to mammals. It is one of the most significant of human behaviours, occupying roughly one-third of our lives. Although the exact functions of sleep are still unknown, it is clearly necessary for survival since prolonged sleep deprivation leads physical and cognitive impairment, and finally death. Sleep disturbances occur in virtually all psychiatric illnesses and are frequently part of the diagnostic criteria for specific disorders.
There are four stages of sleep. Stage one, most related to wakefulness, refers to the period of falling asleep and is characterised by short alpha waves. You then progress into stage two, characterised by higher amplitude and lower frequency waves on EEG recordings. In stage three large slow delta waves are occasionally observed – this is when the person is falling into a deeper restorative state. Delta waves are the largest and slowest and they characterise stage four sleep. When stage four has been reached one stays there and then goes back through to stage one. Stage one is not the same the second time around; when falling asleep the first stage one does not produce Rapid Eye Movement (REM) but the subsequent stage ones do. A night slumber is then spent cycling back and forth between stages one and four; as the night progresses more time is spent in REM stage one and less and less in stage four.
There are two kinds of sleep, non-rapid eye movement (NREM) and rapid eye movement (REM). There are two general theories of sleep, the recuperation theory and the circadian theories; the recuperation theory holds the view that being awake all the time disrupts homeostasis of the body; homeostasis refers to the internal stability of the body. The circadian theory views sleep as being necessary to keep the body inactive in the periods of the day when they are not active and do not need to be active (when being active may risk survival as humans cannot see in the dark and risk being attacked then). Circadian theories rely on the fact that our bodies (and circadian rhythms) adjust themselves to the light and dark. However there have been experiments that demonstrated that even with no environmental cues of light and dark humans and rats would develop a slower but still evident circadian rhythm, which suggests an inherent biological clock.
It is perhaps unsurprising that large internal changes in the structure of sleep have been detected in adolescence; so many changes biological, physical and psychological are happening at this time, the changes in sleep patterns are reflective of the biological changes in the brain. During adolescence sleep, researchers have found big changes in non-rapid eye movement (NREM) sleep EEG. This change during adolescence is so significant that it is more than the decline in the first fifty years of life. This decline in adolescence is accompanied by behavioural changes and changes in the distribution of sleep during twenty-four hours; many parents of adolescents will be familiar with these changes in sleep patterns; adolescents stay up later and are sleepy during the daytime. Clearly, this is happening alongside the social changes, the evenings being more adult holding more fun whereas daytime is less cool being at school. This is not just due to teenage stroppiness. Sleep researchers such as Fienberg and Huttenlocher have argued that the brain actually goes through a profound re-organisation during adolescence. Huttenlocher was one of the first to discover that synaptic density decreases across adolescence and to suggest that this neural re-organisation could be needed for normal cognitive development. Adolescents have very different brain wave patterns, NREM waves are taller and slower whereas REM are shorter and smaller, more akin to waking they are accompanied by bursts in REM.
NREM characterised by delta waves declines most notably during adolescence and continues its decline, although at a much slower rate, across the lifespan. In children around the ages of three to five years of age, there is usually abundant, high amplitude delta wave sleep, which is impervious to noise and external disruptions. This is the “restful” component of sleep making you feel rested, and ready for a new day after sleeping. Beginning in early adolescence, there is a gradual decline in delta wave sleep. Delta waves are interesting because they are predominantly found over the prefrontal cortex, which is involved with higher executive functions such as planning, decision-making, memory and many more vital cognitions.
Feinberg, in the seventies, observed that delta waves increase sharply in the
first year of life, levelled off in childhood then decreased in adolescence. The sharp increase in the early years of life is not surprising as after birth there is a huge increase in cortical connections as the brain is developing and forming. What then is responsible for the sharp decrease in delta waves (NREM) during adolescence? Recent evidence suggests that it is not a simple decline in delta NREM but rather NREM behaves ‘homeostatic ally’. Homeostatically refers to the observation of increasing incidence of delta EEG waves with increasing waking periods prior to sleep, suggesting that changes during waking brain activity need to be consolidated and corrected during the restful recuperative delta wave sleep. Feinberg interpreted such findings as evidence for the recuperative theories. The decline in delta NREM slow wave sleep also could be associated with the first sign of the human nervous system ageing. Further discoveries have indicated that circadian rhythms in mammals are driven by ‘‘pacemakers’’ in the brain, which consist of thousands of neurons in a structure called the hypothalamus. There is evidence of a large shift in circadian rhythms during adolescence. When compared to younger children adolescents have a delay in their circadian rhythm, i.e. specific behaviours seem to occur at a later time of day. The delay in bedtime is an indicator of this phase delay. A morningness/eveningness survey, which documented children’s preference for the timing of activities, found in girls that increasing preference for eveningness was related to pubertal development.