DEFINITION, REGULATION, SLEEP ARCHITECTURE, AND EFFECTS

Sleep is a physiological state of rest characterized by the temporary suspension of consciousness, along with a reduction in voluntary motor activity. During sleep, the brain undergoes changes in its electrical activity patterns, and several important physiological processes take place, including memory consolidation and physical and mental restoration. Good-quality sleep is essential for the healthy functioning of the body. For this reason, it should always be protected, especially during periods of stress, pain, or illness, so that the body has the optimal conditions to cope with these challenges.

How does sleep work?

Sleep is divided into cycles lasting approximately 90 minutes. Each cycle includes REM (Rapid Eye Movement) sleep and Non-REM (NREM) sleep. Once one cycle ends, the next begins, repeating continuously until awakening.
The figure below illustrates a complete sleep cycle, from an NREM phase through to a REM phase, passing through three stages, the third of which corresponds to deep sleep. The following two figures illustrate the overall structure of sleep, commonly referred to as sleep architecture.

Figure 1. NREM–REM sleep cycle showing the successive stages. This cycle repeats every 70–110 minutes, for a total of 3 to 5 cycles during a typical night’s sleep (Adapted from Lavigne et al., 2020).

Figure 2. Sleep cycles (I–IV) and their corresponding NREM–REM stages (horizontal black boxes). During the first third of the night, slow-wave sleep (N3 or deep sleep) predominates. During the final third of the night, REM sleep becomes more prevalent. MT: Movement Time; WT: Wake Time (Adapted from Lavigne et al., 2020).

Each stage is associated with different physiological processes, which are described below.

1. REM Sleep: REM sleep occurs repeatedly throughout the night, with each REM period becoming progressively longer as sleep advances. On average, the first REM stage lasts around 10 minutes, while the final REM stage may last up to 90 minutes. During REM sleep, the following processes occur:

• Rapid eye movements: These movements are associated with the internal visual activity that takes place during dreaming.
• Brain activity similar to wakefulness: Brain activity during REM closely resembles that seen while awake in terms of electrical wave patterns. However, the major skeletal muscles are temporarily inhibited, preventing voluntary movement.
• Vivid dreams: Most dreams experienced during REM sleep are particularly vivid, colourful, and emotionally intense due to increased activity in brain regions involved in imagery, emotions, and memory.
• Temporary muscle paralysis: To prevent us from physically acting out our dreams, REM sleep is accompanied by temporary muscle paralysis, known as muscle atonia.
• Changes in heart rate, breathing, and blood pressure: These physiological adjustments help optimise the body’s functions throughout the different stages of sleep.

2. NREM Sleep: During NREM sleep, the body progresses through three distinct stages, each characterised by different patterns of brain activity and physiological function: N1, N2, and N3, the latter also known as slow-wave sleep.

• Stage N1 (Light Sleep): This is the transition between wakefulness and sleep. During this stage, waking up is relatively easy, and it is common to experience sensations of floating, fleeting thoughts, or brief visual images.
  
  
• Stage N2 (Intermediate Sleep): N2 is a deeper stage than N1. During this phase, characteristic brain wave patterns known as K-complexes and sleep spindles appear. K-complexes are bursts of brain activity thought to suppress responses to external stimuli—in other words, the brain becomes less responsive to the outside environment.
  Sleep spindles are brief bursts of high-frequency brain activity believed to play an important role in memory consolidation, closely linked to the formation of new neural connections and the strengthening of existing ones.
  During N2, the body relaxes further, with reduced muscle activity and a slower heart rate.
• Stage N3 (Slow-Wave Sleep): Also known as deep sleep or delta sleep, N3 is the deepest and most restorative stage of NREM sleep. This stage is essential for physical recovery and growth, as important hormones, including growth hormone, are released. N3 sleep is the most difficult stage from which to awaken. It is also the stage during which sleepwalking and sleep talking are most likely to occur.

What are the effects of sleep?

Reversing fatigue: Sleep allows the body to restore energy levels so it can function effectively throughout the following day.

Biochemical restoration: Sleep promotes synaptic efficiency, lymphatic clearance, protein synthesis, neurogenesis, metabolic restoration (such as glycogen replenishment), and growth. Growth hormone secretion, for example, reaches its peak during sleep.

Immune function: Sleep helps restore and regulate immune function. This is a complex interaction that continues to be the subject of ongoing research.

Memory consolidation: Learning acquired during wakefulness requires sleep to become consolidated. Sleep appears to facilitate both the encoding of new information and the learning of simple tasks, while also supporting behavioural adaptation.

Psychological well-being: Sleep deprivation increases the risk of mood disturbances and depressive symptoms. Although dreaming occurs during all stages of sleep, dreams experienced during REM sleep tend to be the most vivid.

Who regulates sleep?

The Autonomic Nervous System (ANS) controls virtually all automatic functions of the body, including heart rate, digestion, breathing, hormone secretion, and many other physiological processes.
It plays a central role in regulating normal sleep by controlling the transitions between sleep stages and coordinating the body’s physiological functions throughout the night. The ANS consists of two complementary branches: Sympathetic Nervous System (SNS) and Parasympathetic Nervous System (PNS). Together, these systems regulate whether the body’s activity increases, decreases, or remains stable. In general, the SNS predominates during wakefulness, preparing the body for activity, whereas the PNS becomes dominant during sleep, promoting rest, recovery, and restoration. However, activity fluctuates continuously between these two systems throughout the day and night.

For healthy sleep, adequate activation and maintenance of the parasympathetic nervous system is essential. If the PNS is not sufficiently activated, or cannot remain active for long enough during sleep, sleep disturbances and disorders may develop, which will be discussed in a future article.

The nervous system integrates multiple neurological and hormonal signals and, through the autonomic nervous system (ANS), regulates the sleep–wake cycle (SWC). Alterations in these signals lead to responses that disrupt the body’s normal functions. The main regulators of sleep are:

1. Circadian rhythms: These are the body’s internal biological clock, responsible for regulating the daily sleep–wake cycle. Their primary regulator is the natural light–dark cycle.
2. Hormones: Melatonin is a key hormone involved in sleep regulation. It is produced by the pineal gland in response to darkness and helps promote sleep. Other hormones, such as adenosine and stress hormones like cortisol, also influence the sleep–wake cycle.
3. Neurotransmitters: Several brain chemicals, including serotonin, dopamine, histamine, and gamma-aminobutyric acid (GABA), play an essential role in regulating the sleep–wake cycle. Variations in these neurotransmitters affect sleep quality and are themselves influenced by multiple factors, including those described below.
4. Environmental and behavioural factors: Ambient temperature, exposure to artificial light, physical activity, stress, diet, and the consumption of substances such as caffeine and alcohol can all significantly affect both the quality and duration of sleep.

Disruptions to any of these regulators can impair sleep quality. A common example is staying up late. The body is designed to detect, integrate, and interpret changes in environmental light over a 24-hour period. As daylight decreases, the brain interprets this as a signal that it is time to prepare for sleep. However, if we expose ourselves to screens (such as televisions or mobile phones), loud environments and artificial lighting (such as bars or nightclubs), or continue studying late into the night, the brain receives conflicting information. On one hand, the reduction in natural light signals that it is time to sleep (circadian rhythm), while on the other hand, artificial light and ongoing stimulation indicate that we should remain awake. These conflicting sensory inputs can disrupt normal sleep physiology, particularly when they occur repeatedly or over prolonged periods. The same applies when we are physically tired but choose to stay active by working, studying, or exercising instead of resting.

In summary, sleep is an essential physiological state with a well-defined architecture consisting of four stages, during which important physiological changes occur throughout the body. These processes can be disrupted by poor sleep habits, which, over time, may contribute to the development of other health conditions.

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REFERENCES:

Lavigne, GJ., Cistulli, P.A., Smith, MT. (2020) Sleep Medicine for Dentists. An evidence-based overview. 2nd edition, Batavia, IL. Quintessence publishing. ISBN: 9781647240097