What is the circadian rhythm?

The circadian rhythm refers to the physical, mental and behavioral changes that occur in most living things over a 24-hour cycle, mainly regulated by the light or darkness of the environment. It is different from the biological clock, but the two are related in that the biological clock controls the circadian rhythm and related processes in the body.

Conventional circadian rhythm infographic. Image source: elenabsl / Shutterstock.com

Natural circadian rhythm

The circadian rhythm is for regulating sleepiness and wakefulness during the and night. In a normal and healthy person, the rhythm naturally decreases and increases, thereby changing the individual’s sleepiness.

The urge to sleep resulting from the circadian rhythm peaks in most people between 2 and 4 a.m. and increases again in the afternoon between 1 and 3 p.m., although the exact times can vary individually. The strength of these urges depends on the amount of sleep in the previous days and is more intense when a person is sleep deprived. The circadian rhythm also leads to phases of increased vigilance at other times of the .

Physiological processes

The circadian rhythm is regulated by the suprachiasmatic core (SCN) of the hypothalamus in the brain. These cells react to light and dark signals from the environment via the optic nerve of the eyes. The light stimuli then trigger signals from the SCN to other parts of the central nervous system (CNS) to regulate hormones, body temperatures, and other mechanisms that play a role.

For example, when the eyes are exposed to higher light intensity in the morning, the SCN signals to the brain raise body temperature, increase production of the hormone cortisol, and delay the release of the hormone melatonin.

Delayed circadian rhythm in teenagers

It has been observed that teenagers are prone to delayed circadian rhythms compared to children and adults. This means that cortisol and melatonin levels rise and peak later than usual, which can lead to difficulty falling asleep. In some cases, especially when an early morning is required for school or other activities, teens may have difficulty getting enough sleep and are more likely to be sleep deprived.

What Is The Circadian Rhythm?Image Source: Kanyanat Wongsa / Shutterstock.com

Circadian rhythm and jet lag

International travel and time zone changes can disrupt the natural circadian rhythm and lead to disturbed sleep, especially in the first few days as the body adapts to the new environment.

Related sleep disorders

Circadian rhythm sleep disorders involve an abnormality in the timing of a person’s wakefulness and drowsiness that can affect their daily functioning. These disorders include:

  • Delayed Sleep Phase Disorder (DSP): Most common in teenagers and young adults, DSP means feeling tired and waking up at least two hours later than normal.
  • Advanced Sleep Phase Disorder (ASP): Most common in middle and older adults, ASF means feeling tired and getting up at least two hours earlier than normal.
  • Jet lag disorder: occurs when a person changes the time zone quickly (for example, air travel), as a result of which the rhythm does not adapt to the external environment
  • Shift work disorder: occurs when a person’s work schedule interrupts normal sleep, which can lead to shift sleepiness and change sleepiness patterns.
  • Irregular sleep-wake rhythm: includes an irregular sleep cycle, often broken down into several names throughout the , and has been associated with insomnia and sleep deprivation
  • Free running type: also known as the uncompromised type, involves a variable sleep cycle that shifts later each day, regardless of the light in the environment.

The clock in our genes and in every cell in your body | Joseph Takahashi | TEDxSMU 2013Play


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