Life Style

Circadian Rhythm in Humans

Aunwesha Sahani

Circadian rhythm is a biological rhythm which is grounded in time. It consists of physical, mental and behavioral changes occurring within roughly 24 hours.

The term circadian comes from the Latin “circa” meaning around and “diem” meaning day. The rhythm or cycle refers to processes which are endogenous and respond to the environment. This is a natural process primarily responding to light and dark and affecting most living things including animals, plants and microbes. 

Fig: circadian rhythm in human 

Biological clocks and circadian rhythms 

The cycle of circadian rhythms is regulated by biological clocks which are organisms’ natural timing devices. Most of the organs and tissues have their own biological clocks and they are composed of specific proteins that interact with cells throughout the body. 

A circadian rhythm is an effect of a biological clock but not all biological clocks are circadian rhythms. For instance, plants adjust to changing seasons using a biological clock with timing distinct from a 24- hour cycle.

Central control by Master Clock 

A master clock in the brain coordinates all the other biological clocks in living organisms to maintain the sync of circadian rhythms. The master clock in humans is located in the suprachiasmatic nucleus (SCN) which is a group of about 20,000 nerve cells located in the hypothalamus and receives direct input through the eyes. 

The retina has rod and cone photoreceptors for vision and it also contains specialized photosensitive ganglion cells which project directly to SCN. Intrinsically photosensitive retinal ganglion cells (ipRGC) contain photopigment melanopsin and their signals follow the retinohypothalamic tract, leading to the SCN. 

The SCN acts upon the information on the length of day and night from the retina and after interpretation, passes on to the pineal gland. In response the pineal gland secretes melatonin. Production of melatonin is inhibited in the day time and the hormones are produced to keep the human awake. Still in the absence of light, the pineal gland produces melatonin and humans become tired. 

Outside the Master Clock 

The SCN located in the hypothalamus might be the master but more or less independent circadian rhythms are found in many organs and cells of the human body. These are called peripheral oscillators and can be found in the adrenal gland, esophagus, lungs, liver, pancreas, spleen, thymus, skin, olfactory bulb and prostate. 

Researchers suspect that some of the circadian clocks can operate on their own. For further confirmation experiments were performed on mice. The entire circadian rhythm of its body was disabled at first and then the clocks of the skin or liver were activated. 

The results were quite remarkable. Despite the shutdown of all other body and brain clocks, the mice knew what time it was, maybe by getting signals from other body parts. Also, they responded to the light changes as day shifted to night and maintained critical functions like preparing to digest food at mealtime and converting glucose to energy. Only constant darkness stopped the liver function. 

Functions of Circadian Rhythm

Circadian rhythms have many functions and impact many critical aspects of the human body like 

  • Hormone production 
  • Sleep-wake cycle
  • Brain activity 
  • Core body temperature 
  • Cell regeneration 
  • Metabolism 
  • Mental health
  • Measurement and interpretation of day length 

Causes of rhythm disruption 

Changes in our physical and environmental factors can cause the rhythm to be out of sync. Some disruption causing examples are 

  • Mutations or changes in specific genes can affect the biological clock 
  • Jet lag or shift work causes a change in the light-dark cycle
  • Advanced or delayed sleep 
  • Light from electronic devices at night can confuse the biological clock 

Disruption and health hazard 

When the circadian rhythm is thrown off the body’s system doesn’t work optimally. Experiments were performed on mice to know the results of the disruption of circadian rhythms. The disrupted mice were impulsive, had reduced mental stability, disorganized body temperature and gained weight. The study had significant implications for humans. The possible health hazards are 

  • Disturbed body and brain 
  • Obstructive sleep apnea
  • Reduced mental stability
  • Sleeping problems  
  • Impulsive behavior 
  • Premature aging 
  • Type-2 diabetes 
  • Obesity 

Maintaining the rhythm

Many studies highlight the importance of maintaining a circadian rhythm for a healthy lifestyle. The fundamental aspects one must follow to set a circadian clock which would help human to regulate the functionality of human organs are 

  • Light manipulation
  • Maintain sleeping pattern 
  • Regular physical exercise 
  • Regularise on time meal 
  • Embrace seasonal changes 

Aunwesha Sahani 

Biotechnology and genetic engineering discipline 

Khulna University 





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