circadian rhythms Flashcards
Circadian rhythm:
A pattern of behaviour that occurs or reoccurs approximately every 24 hours, and which is set and reset by environmental light levels.
Sleep-wake cycle:
refers to alternating states of sleep and waking that are dependent on the 24-hour circadian cycle.
THE NATURE OF CIRCADIAN RHYTHMS:
Our circadian rhythms are driven by our body clocks, found in all of the cells of the body, and synchronised by the master circadian pacemaker, the suprachiasmatic nuclei (SCN), found in the hypothalamus.
This pacemaker must constantly be reset so that our bodies are in synchrony with the outside world.
Light provides the primary input to this system, setting the body clock to the correct time in a process termed photoentrainment.
In mammals, light-sensitive cells within the eye act as brightness detectors, sending messages about environmental light levels direct to the SCN.
The SCN then uses this information to coordinate the activity of the entire circadian system.
The most familiar of the circadian rhythms subject to this entrainment process is the sleep-wake cycle.
The sleep-wake cycle: part 1
The circadian rhythm not only dictates when we should be sleeping, but also when we should be awake.
Light and darkness are the external signals that determine when we feel the need to sleep and when to wake up.
The circadian rhythm also dips and rises at different times of the day, so our strongest sleep drive usually occurs in two ‘dips’, between 2-4 am and between 1-3 pm (the ‘post-lunch dip).
The sleepiness we experience during these circadian dips is less intense if we have had sufficient sleep, and more intense when we are sleep deprived.
The sleep-wake cycle and homeostatic control
Sleep and wakefulness are not determined by the circadian rhythm alone, but are also under homeostatic control.
When we have been awake for a long period of 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 when most people fall asleep.
Therefore, the circadian system keeps us awake as long as there is daylight, prompting us to sleep as it becomes dark.
The homeostatic system tends to make us sleepier as time goes on throughout the waking period, regardless of whether it is night or day.
The internal circadian ‘clock’ is described as ‘free-running, i.e. it will maintain a cycle of about 24-25 hours, even in the absence of external cues.
The circadian system is, however, intolerant of any major alterations in sleep and wake schedules (e.g. through jet travel, shift work), because this causes the biological clock (and the internal physiological systems that are dependent on this to become completely out of balance.
A case study:
Evidence for a ‘free-running’ circadian rhythm comes from a series of studies conducted by the French cave explorer, Michel Siffre.
On several occasions Siffre has subjected himself to long periods of time living underground in order to study his own circadian rhythms.
While living underground he had no external cues to guide his rhythms - no daylight, no clocks or radio.
He simply woke, ate and slept when he felt it was appropriate to do so.
The only thing influencing his behaviour was his internal body clock, i.e. his ‘free-running’ circadian rhythms.
After his first underground stay of 61 days in the southern Alps in 1962, he resurfaced on 17 September believing the date was really 20 August!
On the second occasion, he spent six months in a cave in Texas. His natural circadian rhythm settled down to just over 24 hours, but with some dramatic variations.
On his final underground stay in 1999, he was interested in the effects of ageing on circadian rhythms (by this time he was 60 years old).
He found that his body clock ticked more slowly compared to when he was a young man, sometimes stretching his circadian rhythms to 48 hours.
Other circadian rhythms:
Core body temperature:
Core body temperature is one of the best indicators of the circadian rhythm.
It is at its lowest (about 36 °C) at about 4:30 am and at its highest (about 38°C) at about 6 pm.
During the normal circadian rhythm, sleep occurs when the core temperature begins to drop, and body temperature starts to rise during the last hours of sleep, promoting a feeling of alertness in the morning.
A small drop in body temperature also occurs in most people between 2 pm and 4 pm, which may explain why many people feel sleepy in the early afternoon.
Hormone production:
Hormone release follows a circadian rhythm.
For example, the production and release of melatonin from the pineal gland in the brain follows a circadian rhythm, with peak levels occurring during the hours of darkness.
By activating chemical receptors in the brain, melatonin encourages feelings of sleep.
When it is dark, more melatonin is produced, and when it is light again, the production of melatonin drops and the person wakes.
EVALUATION/DISCUSSION:
Research support for the importance of light:
Temperature may be more important than light in setting circadian rhythms:
Individual differences:
Research support for the importance of light:
Hughes (1977) tested the circadian hormone release in four participants stationed at the British Antarctic Station.
In February, at the end of the Antarctic summer, cortisol levels followed the familiar pattern, reaching their highest point as the participants awoke, and their lowest point as the participants retired to bed.
However,after three months of continuous darkness,this pattern had changed, with the peak levels of cortisol now being at noon rather than as the men awoke.
This suggests that the extremes of daylight found in polar regions of the world may be responsible for variations in circadian hormone release.
However, other research using scientific communities in the Arctic, who would be subject to similar prolonged winter darkness, found no such disruption of cortisol release patterns.
Temperature may be more important than light in setting circadian rhythms:
Buhr et al. (2010) believe that it is temperature that actually controls our body clock rather than light.
Although light may be the trigger, the SCN transforms information about light levels into neural messages that set the body’s temperature.
Body temperature fluctuates on a 24-hour circadian rhythm and even small changes in body temperature can send a powerful signal to our body clocks.
Buhr et al. found that these fluctuations in temperature set the timing of cells in the body, and therefore cause tissues and organs to become active or inactive.
This suggests that temperature may be more important than light in setting circadian rhythms.
Individual differences:
Research suggests there are individual differences in circadian rhythms.
One is the cycle length; research has found that circadian cycles can vary from 13 to 65 hours (Czeisler et al., 1999).
The other type of individual difference relates to cycle onset - individuals appear to be innately different in terms of when their circadian rhythms reach their peak.
This, according to Duffy et al. (2001), would explain why some people prefer to rise early and go to bed early (about 6 am and 10 pm), whereas others prefer to wake and go to bed later (10 am and 1 am).
