biological rhythms

biological rhythem

A biological rhythm is a cyclical change (occurring in cycles; recurrent) in the way that biological systems behave. These rhythms evolved due to cyclical changes in the environment such as day & night, winter & summer.

types of biological rhythms

circadian rhythm

ultradian rhythm

infradian rhythm

circadian rhythm

Repeats every 24 hours e.g. Sleep/wake cycle or body temperature

ultradian rhythm

Shorter than one day (so occurs many times in a day) e.g. Sleep stages or human alertness

infradian rhythm

Longer than 24 hours e.g. menstrual cycle

exogenous zeitgebers

These rhythms are driven by both internal factors known as(our body clocks) and external factors

what is the circadian rhythm

The main circadian rhythm is the sleep-wake cycle. Nearly all biological organisms have a biological 24-hour day. It is governed by both external and internal signals. Light & darkness are external signals that determine when to sleep & wake. But this rhythm also dips & rises at different times in the day due to our internal body clock E.g. the strongest sleep drive is between 2-4 am & 1-3 pm.

2 factors that drive the circadian rhythm

endogenous pacemakers

exogenous zeitebers

the suprachiasmatic nucleus (SCN)

The SCN consists of a tiny cluster of nerve cells within the hypothalamus. The SCN acts as the 'master clock' as it links to other brain regions that control sleep & arousal. Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the visual area of the cerebral cortex. The SCN lies just above the optic chiasm & receives information about light directly from it. This continues even when our eyes are closed (through eyelids) enabling the biological clock to adjust to changing patterns of daylight whilst we sleep. If our biological clock is running slow (e.g. sun rises earlier than the previous day), the morning light automatically adjusts the clock. The circadian rhythm is therefore put back in step with the world.

pineal gland

The SCN sends signals to the pineal gland which is a small pea like structure behind the hypothalamus.) At night the pineal gland increases the production of melatonin which is a chemical that induces sleep. Melatonin levels decrease in the morning when it becomes light. Insomniacs treated with melatonin find it easier to get to sleep. Melatonin can also be used as a treatment for jet lag.

2 types of circadian rhythm

the suprachiasmatic nucleus (SCN)

pineal gland

supporting evidence for endogenous pacemakers summary

for SCN, Morgan’s hamster

for pineal gland, Schochat sleep study

supporting evidence for endogenous pacemakers - SCN, Morgan’s hamster

Supporting evidence for the SCN comes from a study by Morgan (1995) who removed the SCN from hamsters and found that their circadian rhythms disappeared suggesting that the SCN is the endogenous pacemaker. However, it could be that the operation itself caused damage to other areas that controlled the cycle : further evidence is needed.... So Morgan then transplanted SCN cells from foetal hamsters into these hamsters and the rhythms were re-established. Morgan also transplanted SCN cells from mutant hamsters with shorter cycles (20 instead of 24 hours) and found that the transplanted hamsters took on the mutant cycles. However, the problem with animal studies is that we may not be able to generalise the findings to humans. Therefore, research supporting the SCN as an endogenous pacemaker is flawed.

supporting evidence for endogenous pacemakers - pineal gland, Schochat sleep study

Supporting evidence for the pineal gland comes from a study by Schochat et al (1997): 6 male participants spent 29 hours in a sleep lab (between 7am one day and noon the following day). They spent 7 minutes in every 20 lying down on a bed in complete darkness trying to sleep. The tendency to sleep (sleep propensity) was measured at different times throughout the day. The greatest sleep propensity started in the late evening. The levels of melatonin were measured by taking blood samples throughout the session. The key finding was that there was a positive correlation between the tendency to sleep and levels of melatonin. The strength of this study is that it takes place in the well-controlled environment of the sleep lab. However, a correlation does not imply cause and effect so we cannot say that melatonin causes sleep propensity as sleep propensity might increase melatonin levels. Therefore, research supporting the pineal gland as an endogenous pacemaker is flawed.

exogenous zeitgebers: light

Light is the main exogenous zeitgeber which can reset the internal body clock. This is because receptors in the SCN are sensitive to changes in light. Light doesn't only play a role in the sleep/wake cycle...it also influences bodily processes such as hormone secretion & blood circulation.

One study even found that light may be detected by skin receptors sites on the body even when the same info is not received by the eyes: 15 Ps were woken at various times & a light pad was shone on the back of their knees. The researchers managed to produce a deviation in sleep/wake cycle by up to 3 hours! This shows that light is a very powerful exogenous zeitgeber.

exogenous zeitgebers: social cues

E.g. meal times & social activities which provide info about the time of day. Circadian rhythms begin at about the age of 6 weeks. By about 16 weeks the rhythms are entrained (set). This is likely to be due to the schedules imposed by parents e.g. adult-determined meal and bed times. Jet lag can also be reduced by adapting to local times for eating and sleeping

evaluation of exogenous zeitgebers summary

strength: Siffre’s cave study

weakness: may have been overstated

evaluation of exogenous zeitgebers - Siffre’s cace study

A strength is that there is evidence for the importance of Exogenous zeitgebers from the Siffre case study. Michel Siffre spent long periods of time in a cave with no external clues to guide his circadian rhythm (e.g. no daylight, clocks or radio). He woke, ate & slept when he felt it was appropriate to do so. It was found that without the external cues of lights and clocks Siffre's sleep/wake pattern settled down to 25 hour cycles - rather than the normal 24. Furthermore, there was some dramatic variation, he resurfaced on 17th September 1962 but thought it was 20th August! This suggests that light & other social cues (which he was deprived of) are important in regulating our circadian rhythms, not just the internal body clock.

evaluation of exogenous zeitgebers - may have been overstated

A problem is that the influence of Exogenous zeitgebers may have been overstated. Studies of people who live in the Arctic Circle (where the sun does not set during the summer months) show normal sleep patterns despite prolonged exposure to light. This suggests that there are occasions where Exogenous zeitgebers may have little bearing on our circadian rhythms. However, it could be argued that in these circumstances other external cues such as social cues take over and influence the internal body clock instead of light.

general evaluation of circadian rhythms summary

• important implications in the work place

• individual differences

• use small samples

general evaluation of circadian rhythms - important implications in the work place

A strength of this research is that knowledge of circadian rhythms has important implications in the work place. E.g. workers have reduced concentration in the early hours of the morning as the strongest drive to sleep is between 2-4am This is when more mistakes & accidents occur. In addition, shift workers are 3x more likely to develop heart disease which may be due to the stress of adjusting to different sleep/wake patterns & a lack of good quality sleep in the daytime. Therefore, there are economic implications in terms of reducing illness & how best to manage worker productivity.

general evaluation of circadian rhythms - individual differences

However, a problem with research into circadian rhythms is that there are individual differences in circadian rhythms. It has been found that cycle length can vary from between 13 - 65 hours. Also, some people show a natural preference for going to bed early & rising early (known as larks). Some people are the opposite (owls). Furthermore, there are also age differences in sleep/wake patterns e.g. teenagers' circadian rhythms typically begin 2 hours after adults so current school times mean they have to wake up too early! Therefore the research may not apply to everyone.

general evaluation of circadian rhythms - use small samples and case studies

A problem with research into circadian rhythms is that most studies use small samples & case studies. The sample may not be representative so studies may lack population validity. Even Siffre found that when he returned to the cave at age 60, his internal body clock ticked more slowly than when he was a young man. This shows that even when the same person is involved, there are many factors that affect circadian rhythms so generalisations cannot be made.

circadian rhythm conclusion

Evidence shows that both the SCN & pineal gland are dependent on the light/dark rhythms of the outside world despite being internal mechanisms. Therefore, both Endogenous pacemakers and Exogenous Zeitgebers must play a role in our sleep-wake cycle.

how many stages of of ultradian rhythm

5

what is the ultradian rhythm

One of the features of sleep is that there are several different identifiable types of sleep that we move between throughout the night in a regular pattern. This is an example of an ultradian rhythm, this is thought to be governed by endogenous pacemakers.

In sleep the cycles occur approximately every 90 - 100 minutes and different stages occur within these cycles. During an average sleep period a person will experience 4 to 5 complete sleep cycles. Each cycle consists of 5 stages which were discovered after the development of EEG (Electro encephalogram which measures brain activity)

first stage of of ultradian rhythm

approx 15 mins

Person becomes relaxed & brain activity from different parts of the brain are synchronised. This stage can be regarded as a state of drowsiness. The transition from awake to stage 1 is often accompanied by a hypnogogic state, in which hallucinatory images may be experienced.

second stage of of ultradian rhythm

approx 20 mins

EEG waves become slower and larger but with short bursts of high frequency sleep spindles.

K-complexes are also characteristic of stage 2. These are the brain's response to external stimuli, such as noises in the room. It is still quite easy to be awakened at this stage.

third stage of of ultradian rhythm

approx 15 mins

Sleep deepens as the brain waves slow down

fourth stage of of ultradian rhythm - slow wave sleep

approx 30 mins

Deeper stage of sleep than any of the first 3 stages and is often known as slow-wave sleep (SWS). It is hard to wake someone up in this stage. Growth hormones are secreted at this time. Also sleepwalking & sleep talking can occur as well as night terrors

fifth stage stage of of ultradian rhythm - REM

lasts approx 10 mins in 1st cycle & builds up to an hour by 4th & 5th cycle

Most dreaming occurs here because of increased brain activity, and voluntary muscles become paralyzed otherwise we would act out our dreams! Brain wave activity is the same as when awake.

progression through sleep cycle

After the sleeper has worked through first 4 stages of progressively deeper sleep, he or she reverses the process. This means stage 4 sleep is followed by stage 3 sleep and then by stage 2.

Stage 2 sleep is then followed by REM sleep. After REM sleep, the sleeper starts another cycle from stage 2. Most sleeper's complete about five cycles during a normal night's sleep with progressively less SWS and more REM as morning approaches.

evaluation of ultradian rhythm summary

sleep lab study

sleep partially determined by endogenous pacemakers

individual differences

evaluation of ultradian rhythm - sleep studies

Evidence to support the different stages of sleep comes from a study in which the sleep patterns of S adults was monitored in a sleep lab. Their brainwave activity was measured on an EEG, and evidence was found to support the idea of different stages of sleep. REM activity during sleep was highly correlated with the experience of dreaming (i.e. there was more activity at the time that Ps were dreaming). Brain activity even varied according to how vivid the dream was! Ps who were woken during dreaming reported very accurate recall of their dreams. This shows REM sleep is an important component of the ultradian sleep cycle.

evaluation of ultradian rhythm - individual differences

However, research suggests that there are individual differences in the time & duration of people's ultradian rhythms. In one study Ps spent 11 days and nights in a sleep lab (controlled environment).

Researchers looked at sleep duration, time taken to fall asleep & time spent in each sleep stage. They found large individual differences between Ps. Therefore, the sleep stages proposed may not generalise to everyone.

evaluation of ultradian rhythm - sleep partially determined by endogenous pacemakers

However, this study supports the idea that sleep is at least partially determined by endogenous pacemakers. All Ps were in the same controlled sleep lab environment so it can't be external factors causing the individual differences that were observed. This is contradictory to many people's belief that sleep patterns are due to exogenous zeitgebers e.g. room temperature and noise. Therefore, evidence suggests differences ARE biological (controlled by endogenous pacemakers) & may even be genetic.

infradian rhythm- menstrual cycle

One example of an infradian rhythm is the menstrual cycle. This is controlled by hormones (an endogenous pacemaker).

Oestrogen & progesterone are released into the bloodstream and affect the ovaries & uterus. An average menstrual cycle is 28 days but there are huge variations between women. During each cycle rising levels of ostrogen cause the ovary to develop and release an egg (ovulation). After ovulation progesterone helps the womb lining thicken thus readying the body for pregnancy. If pregnancy does not occur, the egg is absorbed into the body, the womb lining comes away & leaves the body (menstrual flow).

evidence of infradian rhythm summary

can also be influenced by exogenous factors

evolutionary advantages

methodological issues

evidence of infradian rhythm - can also be influenced by exogenous factors

Although the menstrual cycle is an endogenous system, evidence suggests that it can also be influenced by exogenous factors. When several women (who are not taking oral contraceptives) live together their menstrual cycles synchronise. This is due to the influence of pheromones - these act in a similar way to hormones but they effect the bodies of those close to them. In one study, daily samples of sweat were collected from one group of women & rubbed onto the lips of another group of women.

The groups were kept separate but the women's menstrual cycles became synchronised with their

'donor' (the women who provided their sweat). This shows that another person's pheromones (an exogenous factor) can impact the menstrual cycle

evidence of infradian rhythm - evolutionary advantage

It is possible that it is an evolutionary advantage for a social group to have synchronised menstrual cycles so that women will also become pregnant at the same time. This means that all women will breast feed at the same time so that childcare could be shared. This is evident in many female mammals in the wild. (Also remember the Efe tribe in Zaire where all the women breastfeed the babies)

However, a more recent explanation is that having synchronised cycles is evolutionary advantageous for another reason. It may stop all the women getting pregnant from one single male. As women are only fertile for a matter of about 72 hours it is unlikely that a single male could mate with them all at the same time.

This shows there is an evolutionary advantage to exogenous factors influencing the menstrual cycle.

evidence of infradian rhythm - methodological issues

However, there are methodological issues with many of the synchronisation studies conducted. It has been argued that there are many factors that may effect change in a women's menstrual cycle, including stress, changes in diet and exercise, these may act as confounding variables. This means that the supposed pattern of synchronisation may be no more than would have been expected to occur by chance. In addition, the research typically involves small samples of women and relies on Ps self-reporting their own cycle.