achievements, and hindered growth. 7 Children can experience sleepwalking, night terrors, and nightmares. 16 Chronic sleep deprivation is a common issue affecting adolescents. 17 Prolonged lack of sleep in adolescents impairs overall health, behaviour, mood, and academic performance and achievement in this vulnerable age group, particularly during a critical period marked by rapid changes in physical development and emotional regulation. 17 Sleep disorders are not part of normal ageing in older adults; however, their prevalence tends to increase with age, 18, 19 likely to be due to longer sleep latency, lower sleep efficiency, and reduced total sleep duration. 19
Some common sleep disorders include insomnia, 1, 4, 16, 18, 20, 21 16, 19
chronic insomnia, obstructive sleep apnoea, 1, 7, 17, 19, 22, 23 restless leg syndrome, 7, 17 narcolepsy, 1, 7, 17 and circadian-rhythm-disorders. 1
Sleep disorders can lead to significant morbidity and contribute to or aggravate a variety of health conditions. 1 Chronic sleep loss might also be the consequence of a pathological condition, particularly the most common sleep disorder, obstructive sleep apnoea. 14 Chronic sleep disturbances can either be a cause of or be caused by various recognised factors that lead to low-grade inflammation. 4 These factors include circadian disruption, obesity, negative lifestyle choices such as lack of physical activity and poor dietary habits, as well as psychosocial elements like stress, feelings of loneliness, and low socioeconomic status. 4
This brief article explores the physiology of sleep and the various body systems affected by sleep deficiency.
The physiology of sleep
Sleep and wake states are generated by innate neural networks and regulated by the circadian rhythm, a mechanism that governs the body’ s internal clock. 1 The initiation and maintenance of sleep depends on the suppression of ascending arousal systems that promote wakefulness. During the period of wakefulness, extracellular adenosine levels rise, and these increasing levels signal a transition toward sleep. Adenosine activates inhibitory neurons in the ventrolateral preoptic area of the brain, acting as a switch for sleep. 1
Intrinsic biological clocks regulate various homeostatic functions within the body, encompassing sleep and wake cycles. 1 These circadian clocks follow and adhere to rhythms, which are endogenously driven physiological oscillations that have a cycle duration of approximately 24 hours. 1 The primary, or central, circadian clock is known as the suprachiasmatic nucleus and is located within the hypothalamus. 1
Along with the endogenous cycling of transcriptional and translational signals, the circadian rhythm is also affected by external influences. 1 These external influences include exposure to light and darkness, along with social cues. 1 Light signals are detected by the retina, which transmits electrical impulses to the brain, signalling that it is daytime. The retina also recognises the absence of light exposure. 1 In situations of low light 1 and during sleep, 5 the pineal gland secretes melatonin. Melatonin levels rise in the evening and reach their peak in the early morning. 1 Melatonin directly influences the mechanisms that regulate sleep. Similarly, cortisol, a hormone that stimulates activity, increases in response to morning light exposure. 1 During sleep, leptin is secreted by adipocytes, helping to reduce feelings of hunger that could disrupt sleep during this period. 5
A restful night’ s sleep involves a rhythmic, cyclical process that alternates among three stages of non-rapid eye movement( NREM) sleep 1, 4, 19 and a fourth stage called rapid eye movement( REM) sleep. 1, 3, 4, 19 Stage 1 is a light sleep that occurs when a person’ s eyes are closed, and waking is relatively easy. It is often seen as the‘ entry’ into sleep. Usually it makes up about 5 % of a whole night’ s sleep, 1 and is dominated by slow-wave sleep. 3 Stage 2 is a slightly deeper sleep, marked by the stopping of eye movements, 1 a slowing of the heart rate, 1 and a decrease in body temperature. 1 It accounts for roughly 50 % of sleep during the night and is characterised by sleep spindles. 1 Sleep spindles are suggested to support memory consolidation. 6 Stage 3 sleep, also known as deep sleep, slow-wave sleep, or delta sleep, is characterised by high-amplitude slow waves and is mainly seen in the first half of the night. It should generally make up 20-25 % of total sleep. 1 Deep sleep is the stage during which the body undertakes much of its repair work and boosts the immune system overnight. 1 Waking someone during deep sleep is often tricky, and the amount of deep sleep tends to decrease with age. REM sleep follows stage 3 and is marked by rapid eye movements and skeletal muscle relaxation. 1, 3, 4 It mostly occurs in the second half of the night and makes up about 20 % of a typical night’ s sleep. 1 During REM sleep, people experience dreaming, 1, 4 and it is believed that much of memory consolidation happens during this phase. 1 REM sleep has been linked to early learning by supporting the formation of new dendritic spines during early development. 6 A typical night consisting of 8 hours of sleep encompasses roughly five cycles of NREM-REM sleep, each lasting about 90 minutes, which reflects an ultradian rhythm. While NREM sleep is primarily regulated by homeostatic mechanisms, REM sleep is chiefly governed by circadian influences. 4 Preschool-age children who are typically developing and exhibit higher levels of REM sleep tend to retain more learned information than their peers with lower levels of REM. 6 In adults, the characteristics of REM sleep are associated with procedural memory, potentially interacting to facilitate the learning and consolidation of new information. 6 Procedural memories relate to skills and motor sequences, relying on specific neural circuits( such as the basal ganglia and cerebellum) in adults. 6
Conditions associated with sleep deficiency
Lack of sleep has been associated with an increased risk of mortality, 2, 4, 9, 12 and influences numerous health conditions
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