Every night, nearly every person undergoes a remarkable change: we leave waking consciousness and for hours lay immersed in dreams and deep sleep. When we wake, we typically remember little or nothing about the hours that have just passed. However, lately I contemplated just what sleep is and wondered just why we actually need to do it? Surely by sleeping we expose ourself to more danger as a consequence of our vacancy. Surely sleep is an uneconomical way to spend time. Although everyone sleeps, I believe most people would be hard-pressed to precisely define sleep. All organisms exhibit daily patterns of rest and activity that resemble the daily sleep and wakefulness patterns seen in humans. From observing changes in behaviour and responsiveness, scientists have noted the following characteristics that accompany and in many ways define sleep:
- Sleep is a period of reduced activity.
- Sleep is associated with a typical posture, such as lying down with eyes closed in humans.
- Sleep results in a decreased responsiveness to external stimuli.
- Sleep is a state that is relatively easy to reverse (this distinguishes sleep from other states of reduced consciousness, such as hibernation and coma).
Many physiological variables are controlled during wakefulness at levels that are optimal for the body’s functioning. Our temperature, blood pressure, and levels of oxygen, carbon dioxide, and glucose in the blood remain quite constant during wakefulness and change only to accommodate shifts in metabolic demands. During sleep, however, physiological demands are reduced and temperature and blood pressure drop. In general, many of our physiological functions such as brain wave activity, breathing, and heart rate are quite variable when we are awake or during REM sleep, but are extremely regular when we are in non-REM sleep. REM (rapid eye movement) sleep is when the body is in resting state but has high levels of cortical activity, this often results in vivid dreams. In order to prevent us from acting out our subconscious roll plays, all skeletal muscles are paralysed during REM. Non-REM sleep is defined by much lower cortical activity in which we have very little consciousness of our environments. Non-REM sleep is divided into three subsections; N1 (light sleep), N2 and N3 (deep sleep).
For centuries, physicians believed that sleep was a period of brain inactivity, yet research over the last 60 years has shown that the brain remains, at least partially, active during sleep. There is a progressive decrease in the activation or “firing” rate of most neurons throughout the brain as sleep progresses from wakefulness to non-REM sleep. Also, the patterns of neuron firing change from a seemingly random and variable activity pattern during wakefulness, to a much more coordinated and synchronous pattern during non-REM sleep. However, during REM sleep the firing of neurones once again becomes random and much more frequent; this is believed to be the cause of dreaming. In all mammals and many other animals, sleep can be defined in much the same way as sleep is defined for humans. However, there are some notable differences among species. When humans sleep, the entire brain is involved. Dolphins and whales, on the other hand, need to maintain consciousness while they sleep so they can occasionally surface to breathe. In these marine mammals, sleep occurs in only one hemisphere of their brain at a time—allowing for some degree of consciousness and vigilance to be maintained at all times.
One of the possible functions of sleep is to give the heart a chance to rest from the constant demands of waking life. As compared to wakefulness, during non-REM sleep there is an overall reduction in heart rate and blood pressure. During REM sleep, however, there is a more pronounced variation in cardiovascular activity, with overall increases in blood pressure and heart rate. Additionally, changes in blood flow that cause erections to occur in males or swelling of the clitoris in females is characteristic of REM sleep The underlying reason for these considerable neural and physiological variations in REM sleep is currently unknown, and may be a by-product of REM-related changes in nervous system activity or related to dream content.
I personally have only just started to contemplate the true obscurity of sleep, and now find the topic completely fascinating. To think that every night we send our bodies into a temporary state of unconsciousness is amazing, whether it be to conserve energy by reduced body temperature, to give our hearts a slight rest or for an as yet unknown reason, all I know for sure is without it, both the physiological and psychological implications are horrific.