Biological rhythms: circadian, infradian and ultradian rhythms

What are biological rhythms?

Biological rhythms are distinct patterns of changes in body activity that conform to cyclical time periods

What are biological rhythms influenced by?

Influenced by internal body clocks (endogenous pacemakers), as well as external changes to the environment (exogenous zeitgebers)

What is the circadian rhythm?

A pattern of behaviour that occurs or recurs approximately every 24-hours which is set and re-set by environmental light levels

• e.g. 4am = lowest body temperature

• 6:45 = sharpest blood pressure rise

• 14:30 = best coordination

What are some examples of circadian rhythms?

• sleep-wake cycle

• body temperature

• hormone production

How does light affect the circadian rhythm?

• light provides the primary input to this system

• light is first detected by the eye which then sends messages (in relation to brightness) to the suprachiasmatic nuclei (SCN)

• the SCN then uses this information to coordinate the activity of the entire circadian rhythm

What is the sleep-wake cycle?

• a circadian rhythm that dictates when humans should be asleep and awake

• light and dark are the external signals (exogenous zeitgebers) determining when we feel the need to sleep and the need to wake

• the sleep-wake cycle is determined by both the circadian rhythm but also by homeostasis

What is core body temperature?

• an example of a circadian rhythm

• it’s at its lowest at 4am - 36 degrees

• it’s at its highest at 7pm - 38 degrees

What is hormone production?

• hormone release follows a circadian rhythm too

• e.g. release of melatonin

• melatonin comes from the pineal gland in the brain

• peak levels of hormones occurring during the hours of darkness

• so when it is dark, more melatonin is produced and when it’s light again, the production of melatonin decreases and the person wakes

How does homeostasis affect our circadian rhythms?

• homeostasis is a self-regulating process by which biological systems maintain stability while adjusting to changing external conditions

• sleep and wakefulness isn’t only determined by your circadian rhythm

• if you have been awake for a long time, your homeostasis tells us that the need for sleep is increasing due to the amount of energy being used up throughout wakefulness

• this homeostatic drive for sleep increases throughout the day, reaching its maximum in late evening when people finally fall asleep

How does the circadian rhythm and homeostasis affect our need for sleep?

• circadian rhythm keeps us awake as long as there is daylight and prompts us to sleep when it is dark

• the homeostatic system makes us sleepier as time goes on throughout the waking period - regardless of it being day or night

What is free-running?

• our internal circadian clock is described as free-running

• its intolerant of major alterations e.g. change of time zone

• this makes the biological clock become out of balance

Michael Siffre Case Ctudy

• Duration: 2 months

• Location: Southern Alps

• Year: 1962

• Date he resurfaced: 17th September

• Date he thought it was: 20th August

his free running biological rhythm changed to 25 hours

Aschoff and Wever (1976)

• 4 weeks in a WW1 bunker deprived of natural light

• all but one displayed a natural sleep/wake cycle of between 24 and 25 hours

• both Siffres research and the Bunker study suggests that the natural sleep wake cycle may be slightly longer than 24 hours

• however, we train ourselves by exogenous zeitgebers associated with the 24 hour day

Simon Folkard et al (1985)

• studied the influence of environment cues on our internal biological clock

• 12 people living in a dark cave for 3 hours

• went to bed when the clock said 11:45 and out of bed at 7:45

• over the course of the study, the researchers sped up the clocks so an apparent day lasted only 22 hours (unknown to the participants)

• only one participant was able to comfortably adjust to the new regime

suggests the existence of a strong ‘free running’ circadian rhythm that cannot be easily overridden by changes in the external environment

AO3 - research support

P = there is research support for circadian rhythms

EX = this includes the effect of external cues like light on this system

EV = Siffre (1975) found that the absence of external cues significantly altered his circadian rhythm: when he returned from an underground stay with no clocks or light he believed the date to be a month earlier than it was

C = In addition, Folkard  et al (1985) studies the influence of environmental cues on our internal biological clock. 12 people, living in a dark cave for 3 weeks and went to bed when the clock said 11:45 out of bed at 07:45. Over the course of the study, unbeknown to the participants, the researchers sped up the clock so an apparent DAY lasted only 22 hours. Only one participant was able to comfortably adjust to the new regime- suggesting the others found it difficult to adjust to a shorter day. 

LB = this suggests that Siffre’s 24 hour sleep-wake cycle was increased by the lack of external cues, known as exogenous zeitgebers

A)3 - use of case studies

P = studies of the sleep-wake cycle tend to involve small groups of participants

EX = this is ideographic research and difficult to generalise

EV = Aschoff and Wever (1976) studies a small sample of participants who spent 4 weeks in a WW1 bunker deprived of natural light. All but one displayed a natural sleep wake cycle of between 24 and 25 hours. Both Siffres research and the Bunker suggest that the ‘Natural’ sleep wake cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers associated with the 24 hours day.

C = however, it is important to note the differences between individuals when it comes to circadian cycles. Duffy et al (2001) found that ‘morning people’ prefer to rise and go to bed early (6am - 10pm) whereas ‘evening people’ prefer to wake up and go to bed later (10am - 1am). This demonstrates that there may be innate individual differences in circadian rhythm, which suggests that researchers should focus on these differences during investigations

LB = this suggests that individual cycles can vary. There are also age differences in sleep cycles as teenagers circadian rhythms begin 2 hours after adults which means findings should not be generalised.

AO3 - temperature

P = additionally, it has been suggested that temperature may be more important than light in determining circadian rhythms

EV = Buhr et al (2010) found that fluctuations in temperature set the timing of cells in the body and caused tissues and organs to become active or inactive. Buhr claimed that information about light levels is transformed into neural messages that set the body’s temperature

EX = body temperature fluctuates on a 24-hour circadian rhythm and even small changes in it can send a powerful signal to our body clocks

EXT = furthermore, the idea that all human bodies work on a 24 hour cycle is biologiaclly deterministic because it assumes that hormones are the only cause of changes in circadian rhythms, evidence suggests that meal times and travelling can also cause changes to this rhythm as differences in time zones affect ‘normal’ routines, suggesting the influence of light is subdued

LB = this shows that circadian rhythms are controlled and affected by several different factors, and suggests that a more holistic approach to research may be preferable

What is an infradian rhythm?

A rhythm that lasts longer than 24 hours

e.g. menstrual cycle, seasonal affective disorder, hibernation/migration (in animals)

Female menstrual cycle

• regulated by hormones (endogenous pacemakers)

• ovulation occurs halfway through cycle, when oestrogen levels are highest

• after ovulation, progesterone levels increase, ready for possible embryo implantation

• a typical cycle is approx. 28 days

Stern and McClintock (1998)

• ‘syncing up’ menstrual cycles

• 29 women - all with history of irregular periods

• pheromone samples were gathered on cotton pads placed under the arms, from 9 women each at different stages of their menstrual cycles

• they were frozen and treated with alcohol and rubbed on the top lip of the other 20 participants

• day 1 = all 20 got pads from day 1 of the cycle, day 2 = pads from day 2 of the cycle etc.

• McClintock found that 68% of the women experienced changes in their cycle which brought them closer to that of their ‘odour donor’

Seasonal Affective Disorder (SAD)

SAD is an infradian rhythm related to the season

• melatonin is secreted by the pineal gland at night

• longer nights = increased melatonin secretion

• linked to increase in depressive symptoms

Magnussen (2000) suggested a seasonal variation in mood for humans, especially women

AO3 - support for research

P = there is evolutionary value for the menstrual cycle to be synchronised

EX = for our ancestors, it may have been advantageous for females to menstruate together. This would mean that new-borns could be cared for collectively within a social group

EV = research from Stern and McClintock (1988) found that 68% of the women experienced changes in their cycle which brought them closer to that of their ‘odour donor’.

C = however, Schank (2004) argued that if there were too many females cycling together within a social group, this would produce competition for the highest quality males. For this perspective it would be better to not have synchronised menstrual cycles

LB = this makes us question the validity of the evolutionary perspective as there is contradictory arguements

AO3 - criticisms in synchronisation studies

P = However, criticisms have been made of early synchronised studies and the methods employed in them.

EX = Researchers argue that there are many other factors that may effect change in a women's menstrual cycle and the exogenous zeitgebers, such as light, may be more influential than previously thought

EV = For example, Reinberg (1967) examined a woman who spent 3 months in a cave with only a small lamp to provide light. Reinberg noted that her menstrual cycle shortened from the usual 28 days to 25.7 days.

EXT = In addition to this, Stern and McClintock's experiment used a limited sample of only 29 women, as do many other experiments into Infradian rhythms. These small sample sizes prevent the results from being reliably generalised to the wider population because they are too ideographic to conclude that there is a global pattern.

LB = This result suggests that the lack of light, an exogenous zeitgeber, in the cave affected her menstrual cycle, and therefore demonstrates the effect of external factors on Infradian rhythms.

AO3 - practical application - SAD

P = There is also supporting research of the role of melatonin in SAD.

EX = Terman found that the rate of SAD is more common in Northern countries where the winter nights are longer. For example, he found that SAD affects roughly 10% of people living in New Hampshire and only 2% of residents in southern Florida.

EX = These results suggest that SAD is in part affected by light (exogenous zeitgeber) that results in increased levels of melatonin.

EXT = furthermore, a strength of this idea is that it increases our knowledge of how to treat people who suffer from SAD, for example, using phototherapy. The lightbox stimulates very strong light which is thought to reset melatonin levels, and has been found to relieve symptoms for 60% of people

LB = The research into SAD has helped relieve the symptoms of depression and improve the lives of many people suffering from low mood and fatigue.

Ultradian rhythms

ultradian rhythms last fewer than 24 hours

e.g. human sleep patterns/meal patterns - eating 3 times a day

Sleep stages

• the pattern alternates between REM and NREM

  • REM = rapid eye movement

  • NREM = non-rapid eye movement

• consists of 5 stages of 90 mins

• a complete cycle consists of a progression through stages 1-4 and then the cycle starts all over again

• most people have 5 cycles a night

• using EEg (electroencephalogram) scanners to measure the electrical activities within our brains

Human sleep stages

• Stage 1 = light sleep

  • muscle activity slows down

  • occasional muscle twitching

• Stage 2

  • breathing pattern and heart rate slows

  • slight decrease in body temp

• Stage 3 = deep sleep

  • brain begins to generate slow delta waves

• Stage 4 = very deep sleep

  • rhythmic breathing

  • limited muscle activity

  • brain produces delta waves

• Stage 5 = REM

  • brainwaves speed up and dreaming occurs

  • muscles relax and heart rate increases

  • breathing is rapid and shallow

Waves

• alpha waves are associated with light sleep which progress to theta

• delta waves are associated with deeper sleep

• synchronised = recognisable waveform (alpha, theta and delta), often detected during sleep

• desynchronised = no pattern. usually a mix of all 4. often detected when someone is awake

Meal patterns

most humans eat three meals a day and appetite rises and falls because of food consumption

AO3 - evidence for sleep stages

P = there has been a number of studies that have supported the different stages in sleep

EX = this research emphasises how brain waves vary in the stages and how brain activity can change from synchronised to desynchronised depending on the stage of sleep

EV = Dement and Kleitman (1957) monitored the sleep pattern of 9 adults using EEG’s. REM activity during sleep was highly correlated with the experience of dreaming and brain activity varied according to how vivid dreams are.

C = however, whilst Dement and Kleitman’s research had high levels of control on extraneous variables, such as caffeine and alcohol intake, they used a very small sample which milits the generalisability of the study and the validity of the conclusions drawn from it

LB = nevertheless, replication of this investigation have noted similar findings which increases the validity and support for the sleep stages

AO3 - individual differences in sleep cycles

P = it should be noted that there are individual differences when it comes to the stages of sleep

EV = Tucker et al. (2007) found significant differences between participants in terms of the duration of each stage, particularly stages 3 and 4

EX = this emphasises the importance in considering external factors, such as genetics or the environmental context of an individual, as factors like a family history of insomnia or drinking coffee could affect the brain activity and wave patterns during the different stages of sleep

EXT = in addition, a case study of Randy Gardener who remained awake for 264 hours. After this experience, Randy slept for just 15 hours. The nights following the experiment he recovered 70% of stage 4 sleep, 50% of his REM sleep and very little of the other stages

LB = this demonstrates that there may be innate individual differences in ultradian rhythms and they are not as fixed as first thought. This means that it is worth focusing on these differences during investigations into sleep cycles