Biological Rhythms - The Circadian Rhythm

Question 1

The Circadian Rhythm

Answer 1

The circadian rhythm is a 24-hour biological rhythm, often known as the ‘body clock’ which is reset by levels of light. The word ‘circadian’ stems from the Latin: ‘circa’ (meaning ‘around’) and ‘diem’ (meaning ‘day’).

Question 2

Examples of Circadian Rhythms:

Answer 2

1. The Sleep-Wake Cycle
2. Body Temperature

Question 3

1. The Sleep-Wake Cycle

Answer 3

The sleep-wake cycle is an example of a circadian rhythm, which dictates when humans and animals should be asleep and awake.

The exogenous zeitgeber light provides the primary input to this system, meaning that it is the external cue for sleeping or waking up. Light is first detected by the eye, which then sends messages concerning the level of brightness to the suprachiasmatic nuclei (SCN), which is the endogenous pacemaker of the sleep-wake cycle. The SCN then uses this information to coordinate the activity of the entire circadian system.

However, sleeping and wakefulness are not determined by the circadian rhythm alone, but also by homeostasis. When an individual has been awake for a long time, homeostasis signals the body that there is a need for sleep due to energy consumption. This homeostatic drive for sleep increases throughout the day and reaches its maximum in the late evening, when most people fall asleep.

Question 4

2. Body Temperature

Answer 4

Body temperature is another circadian rhythm as it fluctuates over a 24-hour period. Human body temperature is at its lowest in the early hours of the morning (36 degrees Celsius at 4:30 am) and is at its highest in the early hours of the evening (38 degrees Celsius at 6 pm).

Sleep typically occurs when the core temperature starts to drop. The body temperature starts to rise toward the end of a sleep cycle promoting feelings of alertness first thing in the morning.

Question 5

Other examples

Answer 5

Other examples of the circadian rhythm include heart rate, metabolism and breathing. These follow a similar pattern of temperature since they match our patterns of activity. Sensitivity to drugs, release of hormones, and pain sensitivity also vary systematically throughout the day and although we are not aware of them, we may be aware of their consequences.

For example, blood clotting also shows a circadian rhythm, peaking in the morning, which is when heart attacks are most common while the onset of birth labour is common at night, when the female hormone prolactin reaches its peak.