Along with air, food, and drink, sleep is one of those things without which human life is unthinkable. It's a natural, habitual, and vital process. We sleep regularly to rest, feel alert, and energized. Sleep is an essential part of our lives, but how much do we really know about how we sleep?
Falling asleep and waking up
During sleep, vital processes naturally slow down, reducing catabolism. This provides a kind of rest for the body as a whole, including consciousness and thinking. Sleep is periodic and correlates with natural light. This means this process also helps the body adapt to the day-night cycle.
Sleep doesn't occur instantly; it's preceded by specific states: drowsiness (the desire to sleep) and the onset of sleep itself. Before sleep, there's a gradual decrease in brain activity, cardiac function (through a slowing of the heart rate), and sensory systems.
Emergence from sleep is also not an instantaneous process. Awakening and the activation of thought occur gradually. This process primarily depends on the phase of sleep (REM or NREM) during which awakening occurs.
Cycles and stages
But it's not only the previously noted states (pre-sleep onset and subsequent awakening) that are characterized by uneven progression. In fact, all sleep is dynamic and divided into distinct periods with specific patterns of brain activity.
The sleep process is divided into sequential stages, combined into a cycle . This cycle forms the basis of sleep and repeats itself over and over again. The duration of the individual stages within it varies slightly. The order of these stages, however, remains constant: four stages of slow-wave sleep and then one final stage of REM sleep.
During the first cycle of sleep, the proportion of REM sleep is lowest. However, in subsequent cycles, it increases, offset by a reduction in the duration of slow-wave sleep.
Research shows that a full night's sleep is one in which five full cycles have passed.
By the way, the schedule of changes and duration of stages is called a hypnogram .
Slow wave sleep phase
Non-REM and REM sleep are distinct sleep phases characterized by distinct brain activity. Non-REM (slow-wave, or orthodox) sleep is conventionally divided into four stages:
1. First comes drowsiness , so to speak, a semi-conscious state.
2. Then comes light sleep , during which consciousness gradually “switches off,” meaning a decrease in brain activity. At this stage, sleep is most light, and a person can easily awaken.
3. The third can be described as a kind of falling asleep within a dream , a transition to deeper sleep.
4. The fourth stage of the slow wave sleep phase is that very sound, deep and healthy sleep .
As the description of the cycle suggests, slow-wave sleep stages account for the majority of the entire process. Therefore, the slow-wave sleep phase is crucial for restoring the body's energy . If it is interrupted (that is, if awakening occurs during one of the slow-wave sleep stages), the person waking up will feel sleep-deprived, tired, and exhausted.
REM sleep phase
In addition to the stages of slow-wave sleep, the main cycle also includes the rapid eye movement (REM) phase (also known as rapid-wave sleep or paradoxical sleep). This phase is also named for the characteristic rapid eye movements with closed eyelids that occur during this period. The main characteristic of the REM phase is that brain activity during this period is significantly higher, almost as intense as during wakefulness.
This level of conscious and cognitive activity is due to the fact that this phase of sleep is responsible for vivid, memorable dreams . This period can be described as a unique process of interaction between the conscious and subconscious minds, an exchange of information between them. It is the rapid eye movement (REM) phase of sleep that allows the brain to process information received throughout the day (or the preceding waking period).
Waking up during REM sleep is much better; awakening during this period will be easy. You'll feel refreshed and rested . You'll also be almost certain to remember your dream. And often, the dream will be incomplete, as you awaken during REM sleep, that is, while you're dreaming.
Dreams, or visions, are a complex and not fully understood physiological process that deserves a separate discussion.