How deep sleep affects a person’s well-being and how long this stage should last

What is deep slow sleep and how long should it last? Delta sleep is one of the phases of night rest, occupying a fifth of it. During this time, all cells are restored, and the brain processes the information accumulated during the day. It is difficult to overestimate the significance of this stage, since the physical and intellectual abilities of a person depend on it.

What is deep sleep

Night rest accounts for about a third of a person’s life, 7-8 hours a day. This physiological process contributes to the recovery of the body and goes through 4 or 5 successive cycles of alternating fast and slow phases.

The first one (it is paradoxical) takes up to 15 minutes. The second – orthodox or slow sleep – lasts about an hour and a half, occurs immediately after falling asleep, has 4 stages. The last one has the greatest effect on the body, the fourth – deep or delta sleep.

The importance of deep sleep

Why is the delta phase important in the night rest process? During the day, the brain receives and processes a huge amount of various information, and its memorization is carried out in the delta phase. That is, the effectiveness of training and the level of intellectual development directly depend on the quality and duration of deep sleep. In addition to transferring the knowledge gained from short-term memory to long-term memory, physiological processes are of great importance.

In the course of scientific research, it was found that the maximum relaxation of the muscles is noted in the deep stage. At the same time, catabolism slows down and anabolism is activated – the restoration of body cells. Toxins and other harmful waste products are removed from it, and immunity is increased.

Thus, a person fully rests precisely during the delta sleep period. A change in its duration or failure of the entire cycle leads to chronic fatigue, drowsiness, a weakening of the immune system, and a decrease in intellectual abilities.


Slow and REM sleep alternates cyclically throughout the night. Falling asleep begins with the first, orthodox phase. It lasts about one and a half hours and takes place in four successive stages:  

  • Decreased alpha rhythm on the EEG, the appearance of low-amplitude theta rhythms. At this time, a person is in a state of half- sleep , which may be accompanied by the appearance of dream-like hallucinations. Thought processes continue, manifested in the form of dreams and reflections on the events of the day. Often there is a solution to pressing problems.
  • The electroencephalogram registers the predominance of theta waves, as well as the occurrence of characteristic increases in rhythm – “sleepy spindles”. At this, the longest, stage, consciousness turns off, the threshold of perception rises, but it is still possible to wake up the sleeping person.
  • The appearance of high-amplitude delta waves on the EEG. In the third phase of slow wave sleep (5 to 8% of the total duration), they take less than half the time. As the delta rhythm prevails, the deepest delta sleep occurs.
  • In the fourth phase, which accounts for up to 15% of the night’s rest, consciousness is completely turned off, it becomes difficult to wake up the sleeping person. This period accounts for most of the dreams, while the likelihood of manifestations of disorders (somnambulism, enuresis , nightmares) increases . 

Orthodox sleep is replaced by rapid sleep, the ratio is approximately 80% and 20%, respectively. In the paradoxical phase, a characteristic mobility of the eyeballs is observed, if the sleeping person is awakened, he will remember a vivid dream of the sleep phase. The EEG shows electrical activity close to the state of wakefulness. Morning awakening occurs after 4 or 5 full cycles in the “fast” stage.

Normal duration

What is the rate of deep sleep? Its duration and quality are determined by the individual characteristics of the human body . One needs a 4-hour rest, another will need at least 10 to get enough sleep. The duration is also influenced by the age of the sleeper: in childhood it is up to 9-10 hours, in youth and maturity – about 8, and by old age it is reduced to a quarter of a day. The average ideal sleep time is 7 or 8 hours, and the rate of deep sleep for an adult is determined by the percentage of phases. 

If we take 8 hours of sleep as a basis, the duration of the deep period in a healthy person will average 20%. That is, in general, it will take at least 90 minutes, and each of 4-5 cycles will take 20-25 minutes. With a decrease or increase in the night’s rest, the time of each phase decreases or increases, respectively. However, their ratio in percentage terms does not change, and the body is fully restored. 

Processes in the body

The electrical activity of the brain is described in the appropriate section on the structure of sleep. And how do all the phases manifest themselves physiologically? At the beginning of falling asleep, the muscles relax, the pressure and body temperature decrease, and breathing slows down. In the second period, these indicators increase, but it is still possible to wake up a person, despite the partial shutdown of consciousness and an increase in the threshold of perception of external stimuli.

The deep phase, combining the 3rd and 4th stages, is normally characterized by complete muscle relaxation and a slowdown in all metabolic processes. It is difficult to wake up, and physical activity indicates the presence of disorders.

Reasons for violations

Sometimes life circumstances require a reduction in the time of deep sleep (examination session or time pressure at work). A short-term increase in physical activity or mental activity is quickly compensated. But if the duration of this phase decreases over time, chronic fatigue appears, memory deteriorates, and somatic diseases develop.  

The reasons may be:

  • psycho-emotional overload, stress;
  • diseases of internal organs, nervous or endocrine systems;
  • forced awakening at night (with prostatitis to empty the bladder);
  • arterial hypertension.

All these conditions require seeking medical attention and treatment, since delta sleep is vital for a person.

How to normalize deep sleep

The deep sleep phase should be at least 20% of the total sleep. If there are chronic feelings of lack of sleep, weakness and fatigue, it is time to think about how to increase the total sleep time. It is important to follow the regime, try to adhere to the chosen time for falling asleep and waking up. Physical activity during the day and a relaxed atmosphere in the evening, combined with a light dinner, also help to normalize sleep.

Good results are obtained by taking herbal teas or preparations based on melatonin, if the disorder is not associated with physiological pathology. In some cases, medication is indicated, as prescribed by a doctor. For this purpose, sedatives and hypnotics, calcium, magnesium and multivitamin complexes are used. 


Delta sleep plays an important role in the recovery and relaxation of the body, affecting performance, well-being and quality of life. Any deviations from the norm in the form of lack of sleep, memory impairment and chronic fatigue must be treated without fail.

Compliance with the sleep regimen and hygiene, a healthy lifestyle, and relaxation techniques are effective prevention.

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