Circadian Rhythms

What are circadian rhythms?

Circadian rhythms are the result of an adaptation to earth’s rotation applying a 24 hour structure on bodily physiology. Humans are diurnally active. The body with its physiology organized around 24 hours is optimally functioning when this rhythm is paced daily by light.¹

Young men walking up stairs in sunlight
Sleep - Sleep hygiene

What is a chronotype?

People differ in their preferred timing of sleep and wake, which is a physiological behaviour known as chronotypes.² Certain genes have been linked to morning-evening preference and genes that generate and regulate sleep and circadian rhythms are important for understanding the individual variances in sleeping behavior.

Night owls

Night owls have a circadian rhythm period above the average. They tend to be more energetic in the evenings and can easily stay up later than average. They also have more difficulties waking up in the morning. Night owls would benefit from a light exposure that helps them wake up early in the morning to match their spontaneous wake up time with their work schedule. They would then also feel sleepy earlier in the evenings.

Morning larks

Morning larks have a shorter circadian rhythm period. They easily wake up early in the morning – which may lead to insufficient sleep if they need to stay up late at night. The right light exposure would help these individuals keep their alertness level a little longer in the evening and get more quality sleep, up until the time they should wake up.

Most people have a circadian rhythm that is slightly longer than 24 hours.

As in other mammals, in humans, the biological clock shows only a small variation between individuals. Most people have a circadian rhythm that is slightly longer than 24 hours which means that it needs to be corrected daily in relation to the solar day³,⁴. Without correction, the circadian rhythm is shifted slightly every day and gradually falls out of phase compared to the 24-hour solar day.

A need for personalized light

It is evident that modern lifestyles vary significantly in relation to the natural light-dark cycle and influence our access to daylight. Eating habits, when, how and if we choose to exercise, travel schedules and work commitments all affect the bodily rhythms. These parameters are individual. People differ and so do their needs. 

The same light environment is registered by the circadian system very differently between individuals.

Light is the most important time cue.

Light is the most important time cue for the circadian rhythm⁴ as it stimulates light sensitive cells (called ipRGCs) in the eye that projects directly to the SCN. The SCN in the hypothalamus is the master clock in the human brain, where light information is used to synchronize the circadian rhythm with our surroundings.

Most of us don’t get enough light to synchronize with the solar day. We also sleep too little. Circadian disruption is associated with short and long term health effects, including immune suppression and increased risk of infection and cancer⁵,⁶.



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  1. Reiter RJ, Rosales-Corral S, Sharma R. Circadian disruption, melatonin rhythm perturbations and their contributions to chaotic physiology. Adv Med Sci. 2020 Sep;65(2):394-402. 
  2. Elise Facer-Childs (2018), accessed on 2022-01-20 
  3. Czeisler, C. A. et al.Stability , Precision , and Near-24-Hour Period of the Human Circadian Pacemaker Published by: American Association for the Advancement of Science Stable URL: Linked references are available on JSTOR for this article. Science (80-. ).284, 2177–2181 (1999).
  4. Sleep Foundation (2022), accessed 2022-01-27 
  5. Foster RG. 2020 Sleep, circadian rhythms and health. Interface Focus 
  6. Evans, J. A. & Davidson, A. J. Health consequences of circadian disruption in humans and animal models. Progress in Molecular Biology and Translational Science vol. 119 (2013) 
  7. Chaix A, Zarrinpar A, Panda S. The circadian coordination of cell biology. J Cell Biol. 2016 Oct 10;215(1):15-25. 
  8. Addison K and Harris J (2019) How Do Our Cells Tell Time?. Front. Young Minds. 7:5. doi: 10.3389/frym.2019.00005