L11 - Biological Rhythms - Circadian Rhythms

Question 1

What are biological rhythms controlled by?

Answer 1

1) Internal mechanisms known as endogenous pacemakers.
2) External changes in the environment - (exogenous zeitgebers)

Question 2

three main biological rhythms governed by endogenous pacemakers and exogenous zeitgebers

Answer 2

• Circadian rhythms - biological rhythm lasting about 24 hours so happen once in 24 hours
• Infradian Rhythms - biological rhythm greater than 24 hours so happen after 24 hours
• Ultradian Rhythms - biological rhythm less than 24 hours so happen more than once in 24 hours

Question 3

Biological rhythms

Answer 3

Distinct patterns of changes in body activity that follow cyclical time periods

Question 4

Endogenous pacemaker

Answer 4

• also referred to as ‘body clocks’; centres in the brain that play a main role in controlling biological rhythms

Question 5

Exogenous zeitgeber

Answer 5

Environmental factors that help synchronise biological rhythms with the outside world; the best example is light which resets the circadian biological clock in the suprachiasmatic neurones.

Question 6

Circadian Rhythms

Answer 6

• One biological rhythm is the 24-hour circadian rhythm (often known as the ‘body clock’), which is reset by levels of light
• 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 body clock to the correct time in a process called photoentrainment.
• In mammals light-sensitive cells within the eye act as brightness detectors, sending messages about the environmental light to the SCN. The SCN then uses this information to coordinate activity of the circadian system.
• Circadian rhythms can be shown through the sleep-wake cycle, core body temperature and hormone production.

Question 7

Sleep-wake cycle

Answer 7

• Light and darkness are the external signals that determine when we feel the need to sleep and when to wake up.
• This rhythm dips and rises at different times of the day so that strongest sleep drive occurs between 2:00-4:00 am and 1:00-3:00pm.
• The sleepiness we experience is less intense if we have had sufficient sleep and more intense if we are sleep deprived.
• 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 (when people go to sleep).

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 will maintain a cycle of 24-25 hours, even in the absence of external cues, as it is free-running.

Question 8

Key research study to support the sleep wake cycle

Answer 8

• Evidence for free-running circadian rhythms comes most a French cave explorer Michel Siffre – who is a specialist in the study of the human internal clock.
• On several occasions he spent long periods of time living underground to study his own biological rhythms. While underground he had no external cues to guide his rhythms – no daylight, no clock, no radio (so no exogenous zeitgebers).
• He simply woke, ate and slept when he felt like it. The only thing influencing his behaviour was his internal ‘clock’ or free-running’ rhythm. After his first underground stay of 61 days in the Southern Alps in 1962, he resurfaced on 17th September thinking the date was 20th August.
• On the second occasion he spent six months in a Texan cave (Siffre, 1975). His natural circadian rhythm settled down to just over 24 hours.
• Therefore on both occasions, his free running biological rhythm settled down to just beyond the usual 24 hours (around 25 hours).
• This study provides support for the idea of the sleep wake cycle being a circadian rhythm and thus happening once in 24 hours.

Question 9

Core body temperature

Answer 9

• lowest (36ºC) at about 4:30 am and highest (38ºC) at about 6:00pm.
• Sleep occurs when the core temperature begins to drop and body temperature starts to rise in the last few hours of sleep, prompting a feeling of alertness in the morning.
• A study to support core body temperature was carried out by Folkard et al. (1977) who demonstrated how children who had stories read to them at 3pm showed superior recall and comprehension after a week compared to children who heard the same stories at 9am.
• Furthermore Gupta (1991) found improved performance on IQ tests when participants were assessed at 7pm as opposed to 2pm and 9pm.
• Interestingly, a small drop in body temperature does occur in most people between 2-4pm, which may explain why people feel sleepy in the afternoon and also perhaps explain the findings of Gupta’s (1991) study

Question 10

Hormone production

Answer 10

Hormone release follows a circadian rhythm. The release of melatonin from the pineal gland is at its peak during the hours of darkness. By activating chemical receptors in the brain melatonin encourages sleep. When it is light again the production of melatonin drops. This means that because melatonin is produced once in 24 hours it is a good example of a circadian rhythm.