Circadian Rhythm


Circadian rhythm was first discovered by Jean Jacques d'Ortous de Mairan in 1729. The experiment was to see the reaction of light sensitive Heliotrope plants with the absence of light. Initially believing the Heliotrope leaves will stay folded with no light, the leaves continue to fold and unfold in a near 24 hour pattern without any external sensual cues. It was then concluded that there is an internal mechanism that is triggered for day or night activities.

Theoretical frame

Circadian rhythms are the natural biological cycle that all living being possesses. It involves a 24 hour rhythmic cycle in blood pressure, urine production, hormonal secretion, alertness, short-term memory and other aspects of cognitive performance. Research shows that alertness peaks early morning and deteriorates as the day progresses reducing to zero during sleep. Body temperature fluctuates and rises during the afternoon and cools towards the evening while secretion of growth hormone peaks during first two hours of sleep and remains relatively dormant during the day. The disruption in the circadian rhythms can make one feel fatigued, sluggish and irritable during the daytime.

The biological clock is developed largely through timing of behavioural events such as social interaction, meals and sleep. Light also plays a major factor in the functioning of the biological clock and experiments show that it is possible to reset the biological clock through exposure of light. When light travels to the photosensitive retina, information is send to a small area of the brain called suprachiasmatic nucleus (SCN) which processes the light input and sends pulse signals to the pineal gland. The pineal gland will accordingly excrete hormones called melatonin which plays a key role in the biological clock by reducing sleepiness. The circadian rhythms in humans appear to be monitored by several internal clocks, but they all share the same central hub which is the SCN.

Types of Circadian Rhythm Disorders/Disruptions

Jet Lag or Rapid Time Zone Change Syndrome

This syndrome consists of symtoms that includes excessive sleepiness and lack of alertness for people who cross multiple time zones as their body clock is not in sync with the environmental cues such as sunrise and sunset.

Shift Work Sleep Disorder

This sleep disorder affects people who work on shifts at night.

Delayed Sleep Phase Syndrome (DSPS)

This is a disorder of sleep timing. People with DSPS tend to fall asleep at very late times and have difficulty waking up in time for work, school, or social engagements.

Advanced Sleep Phase Syndrome

Advanced sleep phase syndrome is a disorder in which the major sleep episode is advanced in relation to the desired clock time. This syndrome results in symptoms of evening sleepiness, an early sleep onset, and waking up earlier than desired.

Non 24-Hour Sleep Wake Disorder

Non 24-hour sleep wake disorder is a condition in which an individual has a normal sleep pattern but lives in a 25-hour day. Throughout time the person's sleep cycle will be affected by inconsistent insomnia that occurs at different times each night. People will sometimes fall asleep at a later time and wake up later, and sometimes fall asleep at an earlier time and wake up earlier.

(Taken from, 2009 [5])

Supporting evidence

Jet Lag

One of the disruptions our circadian rhythm can encounter is jetlag. For many years, people have been living in their own time zone and rarely ever experience more than 1 time zone in the same day until aviation came into our world. Even by sea, jetlag was a rare event. There are other factors which can contribute to circadian rhythm disruption besides jetlag which is called circadian desynchronosis which includes fatigue, malaise, the lack of sleep, lack of motivation, confusion, insomnia and digestive disorders. In today’s world, travel and work shifts contribute to these factors.

It is very common for travelers today to encounter jet lag, which is not a risk if you are not piloting the aircraft. However, as a pilot typically international or long-haul pilots, constantly go through multiple time zones when conducting a flight. In addition to just one flight, they do it for a living which means, multiple time zones, multiple times. When entering into a different time zone, our body’s rhythm and cues supplied by the help of the environment becomes out of sync (jetlag). It is safe to say that any 3-hour difference can upset our body’s biological clock. 3 hours changes our body’s need to hunger and rest in terms of sunrise and sunset. When travelling for example from New York City to Los Angeles, the body retains the cues and rhythm of the point of origin (NYC) even if you set your watch to Los Angeles local time.

East/West Transition (Flying westward) – New York to Los Angeles

Imagine you are on a business trip to Los Angeles (Pacific Standard Time – PST) from New York (Eastern Standard Time – EST) to attend a meeting to sign a deal. So you fly from New York at 8am (EST) and arrived in Los Angeles at 11am (PST). Your body clock would be in a state following EST which is 2pm. Lunch is in an hour so you figure it will be just nice after checking into the hotel and head for lunch at noon. By this time, your body would be hungry already as noon in LA is 3pm in NY. After the meeting, your watch says its 6pm, which means your body is already at 9pm. You suggest dinner but your colleagues in LA say it is too early. Finally, at 7.30pm you meet up for dinner. Your body clock is beyond that already and says its 10.30pm. By this point, you must be starving. You eat a big meal, have a little chat and then head back to the hotel. When you return to freshen up, LA time says its 9pm, so you decide that it is still too early to go to bed. You turn on the TV and watch something and even head out for a night walk in the city. However your body has other plans. The brain has already started to shut down already as it is midnight by your brain’s biological clock. So you decide to go to sleep and sleeping is not a problem. After sleeping enough, you decide to wake up. Based on your biological clock, its 7am, but local time only says 4am. With a lot of free time, you decided to just lie around until its 6am local time. You cannot force anymore sleep, so you reckon it will be nice to amuse yourself to waste time until your meeting which is at 9am.


As flying backwards rewinds your body clock back 3 hours, mealtime therefore comes very late for your body when it is just mealtime locally. Bedtime comes early in the night when the night is still young. However this problem is not drastic as the body’s free rhythm typically runs slightly longer than 24 hours, thus extending the day would agree more readily with your body clock. Exposure to lateness in sunset, dinner and activity helps establish synchrony between the environment and the body clock. The only problem encountered would be waking up too early. The conclusion is that it is not so tough to cope when flying westward because your body will be early for everything which is easier to adjust. The day becomes longer and it gives time for the body to adjust and prepare for the next day.

West/East Transition (Flying eastward) – Los Angeles to New York

Now imagine your business partner who flies to New York this time from Los Angeles for a meeting. Let’s imagine his flight leaves at 8am (PST) from LA, a typical start to the day and arrives at New York City at 4.30pm (EST). His body clock is only at 1.30pm by now. You plan to meet up for dinner at 7pm but at this time, he is barely hungry as his body is still tuned to 4pm. Anyway, he eats a light dinner before returning to his hotel. He watches TV before going to bed but when bedtime came at around 11pm, he doest feel sleepy. His brain still thinks it is 8pm. Knowing he needs sleep for the meeting tomorrow, he tries hard but could not sleep. Finally at 1am, he falls asleep. He is thinking that it is quite late for a bedtime but his body actually aligns correctly with his sleeping pattern which is 10pm. He sets the alarm for 7am to make it to the meeting at 9am. When it was time to wake up, he could hardly open his eyes. He had 6 hours of sleep which is not too bad but he is trying hard to fight his brain which thinks its only 4am. This is perhaps the lowest point of the circadian rhythm so it can be challenging to try and get out of bed during this period.


Firstly, it was difficult to go to bed early as the body is not tuned to shut down so early. The body’s chemical and internal activation levels are not prepared to allow sleep to come early so it is really tough to force an early sleep, unless you are sleep deprived. Secondly, it is even more difficult to force your body to wake up when it is still within the sleep inertia period. When flying eastward, the day was already shortened which makes it difficult for the body to adjust. The body easily adjusts to delays rather than advances. It might take a few days to readjust the body clock while at the same time, the body may respond to the disruption with indigestion, headaches, sleepiness and insomnia until all of the internal rhythms come in unison again.

[Example taken from Caldwell & Caldwell, 2003 [2]]

Refuting evidence

Way forward (to do list)

1. Aschoff Jürgen (1965). Circadian Clocks. North Holland Press Co. (Amsterdam, Holland). 1965.
2. Caldwell JR, J.A. & Caldwell, J.L. (2003). Fatigue in aviation: A guide to staying awake at the stick. Ashgate: Aldershot.
3. Green, R. G. Muir, H. James, M. Gradwell, D. & Green, R. L. (1996). Human Factors for Pilots (2nd Ed). Ashgate (UK),1996.
4. Koukkari Willard, Sothern Robert (2006). Introducing Biological Rhythms. Springer. (NY, USA). 2006.
5. WebMD. (2009). Sleep and circadian rhythm disorders. Retrieved from on 20 January, 2009.

Knowledge Management Space


Wiki of Science Team (contributors to this page)

Authors / Editors

ZiZhanG NGZiZhanG NG


Peer-revisions & comments

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License