What sleep is in our everyday life is probably not worth telling. Some people get enough sleep, some don’t, but one way or another, every person who is tired during the day dreams of a soft pillow. From a medical point of view, sleep is our adaptation, a state that, through certain neurophysiological, chemical, psychological changes, gives us the opportunity to be physically, mentally and emotionally functional the next day. Sleep is a clear sequential process of changing phases, stages of electrical activity of the brain. The stages of sleep are largely determined by the set of our genes and, interestingly, not all animals are endowed with the ability to sleep in the sense in which we are accustomed to perceiving sleep.
First things first
Complete, true sleep with phases and stages is characteristic only of the most developed warm-blooded animals: mammals (which include humans) and birds. The variety of phases and stages, their duration and depth in different animals is amazing. Some people sleep a few minutes a day, with changes in sleep stages occurring within seconds, as, for example, in the case of a giraffe. And in some species of bats, sleep can last up to 20 hours.
The separation of phases began relatively recently. The science of somnology itself began to take shape just over 80 years ago. Alfred Lee Loomis first described the stages of sleep in the mid-30s of the last century, then in 1953 scientists Dement and Kleitman identified the phase of rapid eye movements, and in 1968 all knowledge was combined by Rechtschaffen into one atlas, which was used by all sleep doctors until 2007, when numerous changes occurred. New methods of examining patients were developed. A path has been passed from the application of several electrodes (electroencephalography) with recording of signals in ink on paper to complex computer systems that, in addition to the electrical activity of the brain, make it possible to simultaneously evaluate many parameters of our body.
Quality sleep: what is it like?
In 2021, the US National Sleep Foundation formulated recommendations for sleep quality. They concerned indicators that characterize the continuity of night rest, its structure, as well as the presence of episodes of daytime sleep. We present some standards in table form.
| Index | Norm | Suitable for persons over 65 years of age | Not the norm for people under 65 |
| Sleep continuity | |||
| Sleep latency (speed of falling asleep) | ≤15 minutes | 31-60 minutes | 45-60 minutes |
| Awakenings lasting more than 5 minutes | Up to 2 times per night | Up to 3 times | ≥4 times |
| Time awake after sleep onset | ≤20 minutes | 31-60 minutes or more | ≥41 minutes (≥51 minutes for teenagers) |
| Daytime nap | |||
| Number of episodes per day | 0 (up to 1 for teenagers) | 0-3 | ≥4 |
| Duration | ≤20 minutes for adolescents; not defined for other groups | ≤100 minutes | >100-120 minutes |
But even this numerous list is not able to cover all the important parameters necessary for analyzing the quality of a night's rest.
Human sleep phases
Human sleep can be divided into several stages. The first stage (it can also be called relaxed wakefulness) is shallow sleep. During this period, we are not yet sleeping, but the breathing rate and pulse decrease, and the level of blood pressure and muscle tone also gradually decreases.
The second stage of sleep is a deeper sleep (medium depth), during which a decrease in blood pressure, pulse, and breathing rate continues and further muscle relaxation occurs. At this moment, we stop reacting to minor external stimuli, to the usual noise outside the window or in the apartment. During this phase, twitching of the arms and legs may occur, sometimes involving the entire body, the so-called sleepy twitches (myoclonus).
The third stage (which has now been decided to be combined with the fourth) is called deep sleep, or delta sleep. It is determined by the presence of delta waves detected on the electroencephalogram. This electrical activity in the brain is like slow waves of the sea, gradually rolling onto the shore of our consciousness and plunging us into a world of dreams. The muscles of the body are relaxed, breathing is calm and rare, the heart beats calmly, and even strong, to a certain extent, external stimuli cannot wake us up.
You can also distinguish a phase of sleep called the rapid eye movement phase. As the name suggests, this phase produces eye movements that can be seen in all of us, even through closed eyelids. This dream can hardly be called calm: blood pressure fluctuates, heart rate constantly changes, the electrical activity of the brain is “chaotic”, we see dreams. If a person is awakened during this phase, then most likely he will be able to tell his dream. During the deep sleep stage, we also see dreams, but almost no one manages to remember them.
The stages of sleep and rapid eye movement are combined into a cycle that lasts from 60 to 100 minutes. During the night, 4-6 cycles change, although this parameter directly depends on the duration of our sleep. In the first half of the night, slow-wave sleep predominates, in the second half - rapid eye movement sleep. In the structure of night sleep, stage 1 should occupy about 5%, stage 2 – 50%, stage 3 – 15-20%, rapid eye movement sleep – 20-25%. And about 5-15% of the time allotted for sleep, we are awake.
