Question sent by TERESA OLIVER (Alcoi). EDUARD ESTIVILL and NURIA ROURE answer:
Sleep and waking life are brain functions and are thus subject to changes of the nervous system. Sleep is neither a passive situation nor a lack of wakefulness, but an active state during which changes in bodily functions happen, in addition to significant mental activity to help balance the individual physically and psychologically. There are hormonal, biochemical, metabolic and temperature changes during sleep that are necessary for the correct performance of a human being during the day. To comprehend the process of sleep, we may think of going down a set of stairs. When we close our eyes, while we are resting, we take the first step down to sleep stage 1, also known as somnolence. In stage 1, the body starts a muscle relaxation, breathing becomes uniform and the EEG (electroencephalography) shows slower brain activity than waking time, similar to that showed during REM stage.
Those differences in the EEG are the reason why sleep is so necessary for our body, and just resting is not enough. After some minutes in this stage, we keep going down towards the so-called stage 2, during which brain waves slow down a bit more. Afterwards, we go on to the deepest sleep, called slow-wave sleep of stage 3/4, during which brain waves are already very slow. Strong acoustic or tactile stimuli are needed to wake up, and it is predominant during the first half of the night. Then we go up again towards stage 2, to enter a new physiological situation we call REM stage because its main feature is Rapid Eye Movement. These four stages combined (1, 2, 3/4 and REM) make up a Sleep Cycle, and take around 90 to 100 minutes altogether. These cycles will repeat four or five times during a full night’s sleep.
During the first half of the night more time is spent under deep sleep, and in the second half, stages 2 and REM are predominant. It is important to spend enough time in each sleep stage because each one provides some kind of physical or mental restoration to our body. That is to say, each stage fulfils a specific function to be able to perform correctly during the day.The functions of sleep are still a biological enigma. With the invention of the electroencephalography and the sleep-deprivation studies some theories (maybe complementary) have been postulated.
Adaptive theory is the oldest one, and it suggests that the fact that we sleep at night makes us lie inactive and, therefore, away from danger in a moment of vulnerability against predators. This fact increases our survival rates.
Restoration and recovery theory delves into the use of sleep to regain different biochemical and psychological processes that have been degraded during the previous waking time. While sleeping, we restore very important functions for our body such as tissue repair, protein synthesis and muscle growth. Thanks to these mechanisms, cognitive function is improved and we can fight neurological fatigue. Total sleep deprivation studies in animals show how they lose all their immunity and die in a few weeks time.
Conservation theory follows a different line. It claims that with sleep comes a decrease in body temperature and calorie needs. Sleep represents a cutback in energy costs during the time of the day in which it is most difficult to find any food. Nonetheless, 10% decrease of metabolic activity under the waking levels is probably not enough to account for sleep as natural selection. In the same way, energy saving during eight full hours of sleep in a person would be 120 calories.
Brain plasticity theory is quite more modern. It asserts that sleep correlates to certain changes in brain structure and organization. Sleep is basic for infants, who need an important brain development. Babies sleep thirteen to fourteen hours a day, and half of that time is devoted to REM sleep (deprivation of that sleep stage affects the ability to consolidate learning or developing different tasks).
Every researcher agrees that sleep has a fundamental function, probably related to neuronal integrity and the reshaping of synaptic connections. Thanks to research during these last fifty years, some essential and specific functions of different sleep stages have been discovered. Slow wave sleep has been credited with anabolic and maintenance functions, as well as with the recovery of endogenous substances. It is also credited with the synthesis of the growth hormone (especially during the first years of development and during childhood) and immunity tasks.
Deprivation of REM sleep in a critical moment of development causes long-lasting changes in brain functioning. Likewise, during the first stages of development, it can inhibit growth response of the brain to environmental stimulation in later stages.
REM sleep seems to be involved in the processes of memory and learning, through general activating mechanisms that facilitate protein synthesis. REM sleep deprivation makes it difficult to remember previous and subsequent learning from waking time, but it does not disturb the strengthening of simple learning. Similarly, it has been discovered that certain learning, critical or attentive situations, especially those consolidating new knowledge, and also emotionally important situations, generate an increase in the proportion of REM sleep. In babies, 50% of total sleep time is REM sleep.
Eduard Estivill. Nuria Roure.