Chronotypes and the Cycle of Sleep

The vital role of sleep in health is well established, with evidenced links to cognitive function, cardiovascular health, metabolic health, mood and emotional wellbeing. Therefore, supporting your sleep should be a clear goal within your life.

Supporting your sleep is often discussed in terms of quantity, yet timing and biological rhythm are equally important for maximising your sleep efficiency and quality. Two individuals may both achieve eight hours of sleep, but experience very different levels of energy, focus, and long term health depending on when that sleep occurs. This is where chronotypes and the sleep cycle come into focus.

Developing a comprehensive understanding on how sleep works is fundamental to being able to effectively adjust your lifestyle to support your sleep quality and in turn optimise your daily performance, longevity and healthspan.

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The Cycle of Sleep

Sleep is cyclical in nature, and a full night’s sleep consists of 4 to 5 repeats of this cycle. A full cycle lasts roughly 90–110 minutes and polysomnography has revealed that each cycle is made up of four distinct stages. Notably, sleep architecture shifts from prioritizing deep, restorative sleep (NREM Stage 3) in the first half of the night’s sleep to longer periods of REM sleep in the second half. This is important as short sleep which does not reach the healthy duration required by the body significantly limits REM, reducing its important cognitive contributions.

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NREM Stage 1

This is the initial, lightest stage of sleep, acting as a brief, 1–7 minute transition phase from wakefulness to deeper sleep. It is characterized by the presence of theta waves as brain waves begin to slow, relaxed muscles, and reduced breathing/heart rate. People in this stage can be easily awakened by noises and changes in their environment.

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NREM Stage 2

NREM Stage 2 is a period of light-to-moderate sleep during which the heart rate is further slowed, decreased body temperature is observed, and there are reduced levels of muscle activity. Electroencephalogram studies show the presence of both sleep spindles (short, rapid bursts of brain activity associated with memory consolidation) and K complexes (large slow waves of activity that help protect the state of sleep from disruption) which are both distinct brainwave patterns during this stage of sleep.

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NREM Stage 3

Also referred to as slow wave sleep, NREM Stage 3 is a state of deep sleep that is believed to be the most restorative of the sleep stages as this is when the body repairs tissues, builds muscle, and strengthens the immune system. During this stage, studies reveal the occurrence of delta waves (brain waves with slow frequency and high amplitude), the lowest heart rate and breathing rate seen throughout sleep, and deep muscle relaxation.

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REM

REM is the only stage of sleep in which rapid, darting eye movements occur and also uniquely includes muscle atonia which is a state of temporary paralysis of voluntary muscles, preventing movement. REM is when vivid dreaming and nightmares are most likely to occur as during this stage there is marked high brain activity. Research indicates that REM sleep is crucial for learning and cognitive health. For example, studies have found that disturbances during REM sleep may be associated with the development of the cognitive dysfunction and brain changes seen in patients with Alzheimer's disease. The key role REM sleep plays in brain development is reflected by its increased presence in the sleep of babies who are undergoing massive cognitive change and learning.

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Chronotypes

Chronotypes reflect the body’s natural inclination to sleep and wake at certain times and regulate when you feel most alert or tired throughout the day. Your chronotype is significantly influenced by genetics, however environmental cues such as light exposure can also impact your chronotype to an extent. It is not simply a preference, and plays a key role in helping determine when your circadian rhythm naturally encourages sleep and alertness.

Chronotypes can be divided into various broad categories, however the two most commonly referred to and studied are:

Morning types (Larks)
Individuals who naturally wake early, feel most alert in the morning and whose energy declines throughout the day and into the evening.
Evening types (Owls)
Individuals who naturally experience greater alertness and energy in the evening, and are better suited to later bedtimes.

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Misalignment between Behaviour and Chronotype and the experience of Social Jet Lag

Aligning your sleep behaviour with your natural chronotype supports more efficient progression through the sleep cycles, improving both physical and mental restoration. Misalignment between chronotype and lifestyle can have measurable effects including reduced sleep quality, impaired cognitive performance and mood, increased risk of metabolic and cardiovascular issues, and hormonal imbalances.

Unfortunately, such misalignments are common and are often due to a phenomenon referred to as Social Jet Lag which is the consistent misalignment between an individual's chronotype and their social or work schedule. An example of social jet lag would be a teenager with a later chronotype, such as an ‘Evening type’ who naturally is most alert towards the end of the day and is better suited to later sleep and wake times, being required to wake up early to attend school where they are expected to be energetic, focused and engage with school work.

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Shifting Chronotypes

Interestingly, chronotypes are not static and seem to follow a pattern of shifting across human lifespan - children generally start closer to ‘Morning types’, waking early and energetic, and then through adolescence people typically shift towards an ‘Evening type’ chronotype, before shifting back towards a ‘Morning type’ through adulthood and into older age. As the chronotype shifts across the lifetime it is important to also adapt your sleep behaviour to match it, as this best supports your health and sleep quality.

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Circadian Rhythm

The circadian rhythm is a roughly 24 hour cycle that regulates sleep, hormone release, metabolism, and body temperature. At its core is the suprachiasmatic nucleus, a region in the hypothalamus within the brain, which acts as an internal pacemaker using exposure to light to guide the circadian rhythm. Exposure to darkness causes the photosensitive retinal ganglion cells within the eye to inhibit the suprachiasmatic nucleus, activating signals that triggers the secretion of melatonin from the pineal gland, preparing the body for sleep. On the other hand, exposure to light causes the photosensitive retinal ganglion cells within the eye to stimulate the suprachiasmatic nucleus which in turn leads to the inhibition of the release of melatonin, inducing wakefulness.

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Zeitgebers

Zeitgebers (German for ‘time-giver’) are external cues that help to synchronize the circadian rhythm to the 24hour cycle. Examples of key zeitgebers include exposure to light, when you eat your meals, temperature change, the release of melatonin and being active both in terms of exercise and socially. They are crucial for maintaining healthy sleep-wake patterns and are therefore a great place to start when looking to support your sleep. For instance, carefully managing when you are exposed to light, ensuring your environment is dark on the run up to when you wish to sleep and bright during the day when you wish to be active, can help entrain your sleep cycle to match your desired schedule.

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Conclusion

Sleep follows a cyclical pattern with each of the four distinct phases of sleep holding vital function. While chronotypes and sleep behaviour is largely the result of genetics, zeitgebers support the circadian rhythm to help induce sleep and wakefulness in line with your environment. Natural sleep requirements can differ widely between individuals, as well as across an individual's lifespan, and so it is important to explore what your body’s natural inclination for sleep is and match this pattern as best as possible to support your health and sleep quality.

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