Circadian Rhythm: How It Works
Circadian Rhythm
Circadian Rhythm
Circadian rhythms act as an internal biological clock to regulate body processes and ensure they happen in the right order.1 While circadian rhythms function internally, they are synchronised to the external world, via signals known as zeitgebers that provide information on the time of day; these include sunlight, eating and drinking.1,2 An inability to correctly interpret these zeitgebers in children with neurodevelopmental disorders can lead to abnormalities in circadian rhythm.3
The circadian rhythm is focused on the daily cycle of activity and sleep.1 During the body’s sleep phase, cellular repair, toxin clearance, information processing and memory consolidation occur, which is why any disruption to the circadian rhythm can have a significant effect on a range of regulatory functions.1
The suprachiasmatic nucleus (SCN) within the hypothalamus is the site of the biological clock.4 The SCN helps control the parts of the hypothalamus that regulate the release of hormones such as cortisol and melatonin, meaning that these are also subject to circadian rhythm.1
Melatonin and Circadian Rhythm
Melatonin and Circadian Rhythm
Melatonin (N-acetyl-5-methoxytryptamine) is a natural substance synthesised from the amino acid tryptophan in the pineal gland that is released in the evening in response to SCN signals.1,3 Melatonin has been called the ‘hormone of darkness’ as its synthesis and release are boosted by darkness and inhibited by light.4
The period when the body’s natural melatonin levels start to rise in low light conditions is known as the Dim Light Melatonin Onset (DLMO).5 This differs by individual and is the characteristic of their intrinsic circadian timing.5 Normally the DLMO occurs around 7-9pm in children aged 6-12 years of age and between 7.30-9.30pm in adults.5 Identifying when a patient’s DLMO begins can help optimise sleep management approaches.
The Role of Zeitgebers
The circadian rhythm can be affected by external cues, known as zeitgebers.2 These include:2
Sunlight
Eating
Drinking
Work
Exercise
Chronic Sleep and Circadian Rhythm Disruption
Disruptions to sleep and circadian rhythm can affect emotional responses, cognition, physiology and health.1
Increased risk of:
- Daytime sleepiness and microsleeps.1
- Systemic illnesses, such as cardiovascular disease, cancer, type II diabetes, metabolic abnormalities, infections.1
- Mental health issues, such as depression, altered stress response and psychosis.1
- Chronic insomnia has been associated with a loss of 5.6 million quality adjusted life years per annum, with a greater impact than depression or arthritis.6
Increased:
- Mood changes, irritability and anxiety.1
- Frustration and risk-taking1 – the more disturbed the sleep in adolescents engaging in risk-taking behaviours, the higher the risk of suicide attempts.7
- Lack of empathy.1
- Use of stimulants (e.g. caffeine), sedatives (e.g. alcohol) and recreational drug use.1
Impaired:
- Memory, attention and concentration.1
- Cognitive performance and ability to multi-task.1
- Communication and decision-making.1
- Creativity and productivity.1
- Motor performance.1
The Circadian Rhythm and ADHD
A more recent development is the concept that ADHD may also be associated with alterations in the circadian rhythm.8 Although more research is required, it has been suggested that people with ADHD have an evening circadian preference and that those with ADHD and sleep-onset insomnia have a delayed DLMO compared with those with ADHD but no insomnia or healthy controls.8
Sleep Architecture
Sleep Architecture is the Structural Organisation of Normal Sleep1
Sleep is comprised of two types of sleep that alternate cyclically:1,2
- Rapid eye movement (REM),1,2 when dreaming occurs2
- Non rapid eye movement (NREM):1,2
- Stage 1 (also known as N1) – the lightest sleep and most likely to wake
- Stage 2 (also known as N2)
- Stage 3 – along with stage 4 this is known as slow wave sleep and the two stages combined are also known as N3
- Stage 4 – the deepest sleep.
Sleep is comprised of two types of sleep that alternate cyclically:1,2
- Rapid eye movement (REM),1,2 when dreaming occurs2
- Non rapid eye movement (NREM):1,2
- Stage 1 (also known as N1) – the lightest sleep and most likely to wake
- Stage 2 (also known as N2)
- Stage 3 – along with stage 4 this is known as slow wave sleep and the two stages combined are also known as N3
- Stage 4 – the deepest sleep.
Right-click / long tap to see bigger image
Sleep stages for a normal young adult.
Figure adapted from Carskadon and Dement, 2011.2
Sleep stages for a normal young adult. Figure adapted from
Carskadon and Dement, 2011.2
1. Colten, HR and Altevogt BM. Sleep disorders and sleep deprivation: an unmet public health problem. Washington: National Academies Press. 2006.
Sleep Across the Ages
This sleep architecture differs by age:2
- At birth active sleep accounts for 50% of total sleep time, with NREM Stages 3 and 4 not present but emerging over the first two years of life2
- During adolescence NREM stages 3 and 4 decrease by 40% from the pre-teen years and continues to decline into old age2
- Older adults (from 65 years) tend to have earlier bedtimes and wake times than younger adults.1
This sleep architecture differs by age:2
- At birth active sleep accounts for 50% of total sleep time, with NREM Stages 3 and 4 not present but emerging over the first two years of life2
- During adolescence NREM stages 3 and 4 decrease by 40% from the pre-teen years and continues to decline into old age2
- Older adults (from 65 years) tend to have earlier bedtimes and wake times than younger adults.1
Factors That Affect Sleep Architecture
Circadian Rhythm
If sleep onset is delayed until the peak REM phase of the circadian rhythm i.e. early morning, REM sleep tends to predominate and can even occur at the onset of sleep.2 The circadian rhythm also controls the release of melatonin, which is important in inducing sleep.1
Temperature
Temperature extremes disrupt sleep, with the REM phase most affected.2 Body temperature is lower at night and starts to rise in the hours before waking.1
Circadian rhythms of core body temperature and melatonin in adolescents.
Adapted from Logan and McClung, 2019.3
Sleep Pressure
Sleep pressure or drowsiness also interacts with the circadian rhythm.4 The greater the difference in time between the increase in drowsiness and an individual’s circadian rhythm the higher the sleep pressure.4
The interaction between sleep pressure and circadian rhythm where the green line is normal sleep pressure, the blue line is normal circadian rhythm and the dotted lines are delayed circadian rhythm.
Adapted from Bijlenga, et al, 2018.4
Sleep Disorders
Irregular sleep cycles, with or without missing sleep stages, can be seen in those with sleep disorders.1
Drug Intake
Sleep stages can be disrupted by a number of drugs:2
- Benzodiazepines suppress slow wave sleep
- Antidepressants, such as tricyclic antidepressants and some selective serotonin reuptake inhibitors suppress REM sleep
- Alcohol can increase slow wave sleep and suppress REM early in the night, followed by REM sleep rebound later in the night.2
UK-AGB-ADA-0016 | Date of preparation: January 2024
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