What is deep sleep?

Deep sleep is one of the most important sleep stages, belonging to NREM sleep. It is the slow-wave sleep stage where the brain generates delta waves, and the body enters an intense state of regeneration [1]. During this stage, crucial restorative processes take place, influencing physical and emotional well-being.

Sleep consists of cycles, with one sleep cycle lasting approximately 90 minutes and repeating several times throughout the night [2].

What are the sleep phases?

Sleep phases are divided into NREM and REM. These phases make up the entire sleep structure, and sleep is divided into several stages that constantly repeat throughout sleep [2].

NREM (Non-Rapid Eye Movement) sleep

Phase 1
This is a transitional state from wakefulness to sleep. In this phase, brain activity begins to slow down, and muscle tone decreases [1].

Phase 2
A light sleep stage where body temperature decreases, heart rate slows down, and breathing stabilizes [1].

Phase 3 – deep sleep phase
This phase is the deepest part of NREM sleep. During this time, the body restores tissues, strengthens the immune system, and growth hormone is released [3]. This is the most important regenerative stage.

REM (Rapid Eye Movement) sleep – what's different

REM sleep is characterized by rapid eye movements and active brain activity. The REM phase is important for memory consolidation and emotional processing [4]. Unlike NREM, brain activity in this stage resembles wakefulness.

How do deep sleep phases affect the body?

During deep sleep:

• cell renewal occurs [3]

• the immune system is strengthened [3]

• heart rate stabilizes [1]

• stress hormone levels decrease [5]

If this stage is shortened, sleep deprivation, fatigue, or even insomnia can occur [6].

Conditions determining sleep quality

Sleep quality depends on sleep patterns, environment, and biological processes.

Sleep patterns and their importance for quality sleep

A regular sleep schedule helps maintain circadian rhythm and facilitates the transition to deep sleep [7].

Environmental factors

Optimal temperature and darkness are essential for quality rest. Even slight temperature changes can disrupt slow-wave sleep [1].

Daily habits

Stress and anxiety can reduce the duration of deep sleep, as cortisol levels increase [5].

How to improve sleep quality and get more deep sleep?

Sleep quality depends on sleep patterns, environment, and biological processes. To get more deep sleep, not only sleep duration but also sleep structure and sleep cycle stability are important.

Proper sleep schedule

A regular sleep schedule helps maintain circadian rhythm and facilitates the transition to deep sleep [7]. When going to bed and waking up at the same time, the brain more effectively initiates NREM stages.

Stability of sleep patterns:

• reduces sleep onset latency

• increases the proportion of deep sleep in the first half of the night

• helps prevent sleep deprivation

Even significant time changes on weekends can disrupt the sleep cycle.

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Stress reduction

Stress directly affects cortisol levels, and elevated cortisol can suppress deep sleep [5]. If the sympathetic nervous system is active in the evening, it is harder for the body to transition into a state of relaxation.

Effective measures:

• breathing exercises

• meditation

• light stretching exercises

• limited screen time before bed

Over time, this helps stabilize hormone balance and improve sleep quality.

Lifestyle changes

Lifestyle strongly affects sleep quality. Regular physical activity increases the proportion of slow-wave sleep, but intense training late in the evening can have the opposite effect.

It is also important to:

• limit caffeine after lunch

• avoid alcohol before bed

• maintain stable blood sugar levels

These factors help the body more effectively transition into the deep sleep stage.

Pillow selection

An unsuitable pillow can cause neck tension and micro-awakenings that fragment the sleep cycle. If the head position is not ergonomic, the body remains in partial tension, which reduces the duration of deep sleep.

A suitable pillow should:

• support a neutral spinal alignment

• allow the skin to breathe

• not contribute to overheating

A pillow that is too high or too low can disrupt natural relaxation and reduce sleep quality.

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Insomnia and its impact on deep sleep duration

Insomnia shortens the deep sleep phase and fragments the sleep cycle [6]. Chronic sleep deprivation can increase the risk of cardiovascular diseases and weaken the immune system [8].

Melatonin is a hormone that regulates the sleep cycle and circadian rhythm [7]. In some cases, melatonin supplements may be used for sleep regulation.

Valerian extract is often used as a natural remedy to promote relaxation, although scientific evidence for its effectiveness is moderate [9].

Practical tips for getting more deep sleep

• Maintain a regular sleep schedule [7]

• Limit caffeine and alcohol in the evening [6]

• Reduce stress before bed [5]

• Ensure a suitable sleep environment [1]

Over time, these factors help ensure more deep sleep and better well-being.

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References

1. National Institute of Neurological Disorders and Stroke. (2023). Brain Basics: Understanding Sleep.
2. Harvard Medical School. (2022). Stages of Sleep.
3. Rasch, B., & Born, J. (2013). About sleep's role in memory. Physiological Reviews.
4. Walker, M. (2017). Why We Sleep. Scribner.
5. Padalkar, P., Doshi, P. R., & Padwal, M. (2025). Cortisol and immune function. Cureus.
6. American Academy of Sleep Medicine. (2023). Insomnia and Sleep Deprivation.
7. National Sleep Foundation. (2023). Circadian Rhythms and Sleep.
8. Garbarino, S., et al. (2021). Role of sleep deprivation in immune-related disease risk. Communications Biology.
9. Bent, S., et al. (2006). Valerian for sleep: A systematic review. American Journal of Medicine.