L11 - Biological Rhythms - Circadian Rhythms Flashcards
What are biological rhythms controlled by?
1) Internal mechanisms known as endogenous pacemakers.
2) External changes in the environment - (exogenous zeitgebers)
three main biological rhythms governed by endogenous pacemakers and exogenous zeitgebers
- 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
Biological rhythms
Distinct patterns of changes in body activity that follow cyclical time periods
Endogenous pacemaker
- also referred to as ‘body clocks’; centres in the brain that play a main role in controlling biological rhythms
Exogenous zeitgeber
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.
Circadian Rhythms
- 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.
Sleep-wake cycle
- 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.
Key research study to support the sleep wake cycle
- 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.
Core body temperature
- 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
Hormone production
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.
Evaluation of Circadian rhythms
strengths
- drug treatments
- understand disruption
weaknesses
- small sample
- artificial light
Drug treatments
- One practical application of circadian rhythms is chronotherapeutics (the study of how timing affects drug treatments) 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. This is all due to the fact that by researchers studying circadian rhythms, we are able to help people in the real world.
Affects of disrupting rhythms
- Another practical application of circadian rhythms has given researchers a better understanding of the adverse consequences of disruption of circadian rhythms through desynchronization namely shift work. For instance people doing night shift experience a reduced period of concentration at 6.00a.m. where it is likely that mistakes and accidents are more likely to happen – by knowing this employers can then reshuffle their timetables so employees are less busy at that particular time.
Small sample size
One disadvantage of studying circadian rhythms is the use of case studies and small sample sizes (e.g. Michael Siffre) especially when studying the sleep wake cycle – this questions the generalisability of the studies as well as whether these studies are truly representative of the target population as a whole – for example, in his most recent cave experience in 1999, Siffre observed, at the age of 60, that his internal clock ticked much more slowly than when he was a young man. This illustrates the fact that, even when the same person is involved, there are factors such as age which vary which may prevent general conclusions being drawn about circadian rhythms.
Artificial light
- Early research studies of circadian rhythms suffered from an important flaw when estimating the ‘free running’ cycle of the human circadian rhythm – in most studies participants are deprived of objects that could affect their circadian rhythms such as clocks and daylight. However, they were not deprived of artificial light such as a torch. This means that the participants that were part of these studies still had exogeneous cues albeit artificial. In fact, Czeisler et al., (1999) was able to alter participants’ circadian rhythms to between 22- 28 hours by manipulating the use of artificial lighting alone – this suggests that circadian rhythms may not be so timely after all!
