Sleep is a reversible behavioural state of perceptual disengagement from and unresponsiveness to the environment (Carskadon & Dement in Principles and Practice of Sleep Medicine 2005; p.13).
Early physicians perceived sleep as an intermediate state between wakefulness and death!
We all need sleep – children need 11-12 hours of sleep, adolescents 9-10 hours and adults 7-8 hours of sleep to function safely and be awake in passive situations.
Sleep is complex and is characterised by both physiological and behavioural processes. In sleep there are two distinct states, one is non rapid eye movement (NREM) and the other is rapid eye movement (REM). NREM is made up of 4 stages increasing in depth. Stage 1 or transitional sleep is between wakefulness and the onset of stage 2 sleep, which accounts for 45-55% of total sleep. After approximately 25-35 minutes of sleep we move into sleep stages 3 & 4 which is delta or deep sleep (low voltage signals with high amplitude waves). This deep sleep accounts for about 20% of night. During these deeper stages of sleep the brain rests and few electrical signals go to the executive, planning, logical part of the frontal cortex. REM or dream sleep is similar to wakefulness and is characterised by high voltage but low amplitude waves so our minds are very active but the body is semi-paralysed making sure we do not get up and act out our dreams.
We cycle through all the stages of sleep approximately every 90 minutes with most of our deep sleep being in the first one third of the night. Towards morning our sleep tends to be lighter (stage 2) and there are longer periods of dreaming.
The question remains of why we sleep. We know sleep is important at both a cell and network level in terms of repair and maintenance, endocrine function, energy conservation, environmental and brain adaptation, and in learning (Mignot, PLoS Biology, 2008; 6: 661). Other theories relating to the role of sleep involve information processing (memory improves following sleep) and ongoing plasticity of the brain. Gene expression also appears to be altered with sleep. Whatever the true role of sleep it needs to be valued and adequate time set aside for this very important and safety promoting evolutionary state.