How human beings sleep
When we look at human beings from a certain perspective, it appears that there are two different normal states of being: sleep and wakefulness. Looking closer into these two different states, it becomes clear that sleep is not a homogeneous entity, but it consists of two distinguished phases: REM sleep and Non-REM sleep, and these are as different as sleep is from wakefulness.
Adult human beings spend about two-thirds of their day awake. This state is characterized by different degrees of physical activity or even quiet rest, with muscles being active, mostly in upright position, with brain active and thinking analytically in a very well structured and understandable way, and memory functioning normally. Heart rate and respiration vary according to instantaneous needs, eyes move rapidly to allow visual information coming from the environment.
Non-REM sleep is very different, as we are quiet and recumbent, muscles at resting, but with moving capability spared. Brain activity is at lowest, thought content is structured and analytical, as during wakefulness. Sensory-motor disengagement from the environment is dependent on the sleep depth. There are varying awakening thresholds. Heart rate and respiration are regular and temperature control is similar to that at wakefulness level. Eye movements are absent. By contrast, during REM sleep the body is usually quiet and recumbent, muscle activity is inhibited and normally voluntary muscles are actively kept from contracting, brain is as active and sometimes more so than during wakefulness, but the active centers are different.
The thoughts are bizarre and hyper associative, while memory is impaired. Heart rate and respiration are irregular, body temperature is not well controlled, eye movements are present and rapid, resembling wake time. Human beings are dreaming.
Within that one-third of the time we spend sleeping, one quarter to one third is spent in REM sleep, about two thirds in Non-REM sleep, and around 5%-10% awake. To complicate things a bit more, Non-REM sleep is subdivided into deep sleep and light sleep, with deep sleep taking about one-third of the total Non-REM time. It seems that we have some law of the “one third” working here: one third of the time sleeping, one-third of the sleeping time dreaming, and one-third of the non-dreaming time spent in deep sleep. All these thirds try to find a balance and kind of play a game: when one is ready to sleep, one has some winding down routines, one finds the right position and gradually disconnects from the environment, and finally one enters the kingdom of sleep. All these steps take around ten minutes, and then there is a quick slide into a first period of deep sleep via a short light-sleep phase. When time arrives, there is a resurfacing to almost wakefulness sometimes and a very short episode of dreaming. All this takes about 90-110 minutes and the process restarts with a second cycle, followed by a third one, until a final wake up occurs after about 5-6 cycles normally. The structure of the described sleep cycles changes across the night, with the amount of Non-REM sleep prevailing during the first two-thirds of the night and REM sleep towards morning time. Deep sleep is the prevailing type of Non-REM sleep during the first two cycles and light sleep takes over thereafter.