Lesson 11: Circadian Rhythms

Question 1

Biological rhythms

Answer 1

Biological rhythms are cyclical changes in physiological systems. They evolved because the environments in which organisms live have cyclical changes e.g. day/night, summer/winter etc. There are three types of biological rhythms, circadian, ultradian and infradian.

Question 2

Circadian rhythms

Answer 2

Circadian rhythms are any cycle that lasts 24 hours. Nearly all organisms possess a biological representation of the 24 hour day. These optimise an organism’s physiology and behaviour to best meet the varying demands of the day/night cycle.

Circadian rhythms are driven by the suprachiasmatic nuclei (SCN) in the hypothalamus. This pacemaker (controls the rate at which something occurs) must constantly be reset so that our bodies are in synchrony with the outside world. Natural light provides the input to this system, setting the SCN to the correct time in a process called photoentrainment. The SCN then uses this information to coordinate activity of circadian rhythms throughout the body.

Question 3

Sleep-Wake cycle

Answer 3

Light and darkness are the external signals that determine when we feel the need to sleep and when we wake up. This rhythm dips and rises at different times of the day so that the strongest sleep drives occur between 2:00-4:00am and 1:00-3:00pm.

The release of melatonin from the pineal gland is at its peak during the hours of darkness. Melatonin induces sleep by inhibiting the neural mechanisms that promote wakefulness. Light supresses the production of melatonin.

Sleep and wakefulness are also under homeostatic control. When we have been awake for a long time homeostasis tells us that the need for sleep is increasing because of the amount of energy used up during wakefulness. This homeostatic drive for sleep increases gradually throughout the day, reaching its maximum in the late evening.

Circadian rhythms keep us awake as long as there is daylight, prompting us to sleep as it becomes dark. The homeostatic system tends to make us sleepier the longer we have been awake regardless of whether it is night or day. The internal circadian rhythm will maintain a cycle of 24-25 hours, even without natural light.

Question 4

Advantages of circadian rhythms

Answer 4

+ One practical application of circadian rhythms is chronotherapeutics. The time that patients take medication is very important for treatment success. It is essential that the right concentration of drug is released in the target area of the body at the time the drug is most needed. For example, the risk of heart attack is greatest during the early morning hours after waking. Medications have been developed that are taken before the person goes to sleep but are not released until the vulnerable time of 6:00 am.

Question 5

Disadvantages of circadian rhythms

Answer 5

• Research on circadian rhythms has not isolated people from artificial light, because it was believed only natural light affected circadian rhythms. However, more recent research suggests this might not be true. Cziesler et al. (1999) altered participant’s circadian rhythms down to 22 hours and up to 28 hours by using artificial light alone.
• There are individual differences in the length of circadian rhythms. One research study found that cycles can vary from 13 to 165 hours (Czeisler et al, 1999).
• Another individual difference in circadian rhythms is when they reach their peak. ‘Morning people’ prefer to rise early and go to bed early whereas ‘evening people’ prefer to rise late.
• Studies of individuals who live in Artic regions, where the sun does not set in the summer months, show normal sleeping patterns despite the prolonged exposure to light. This suggests that there are occasions where the exogenous zeitgeber of light may have very little bearing on our internal biological rhythms