Circadian, Infradian & Ultradian Rhythms (AQA A Level Psychology)

Circadian rhythms

• Circadian rhythms (aka the ‘body clock’) operate on a 24-hour cycle which is reset by levels of light

• The sleep-wake cycle is an example of a circadian rhythm

  • Humans and animals will sleep or wake according to this cycle, dependent on the amount of light they are exposed to

  • Environmental events can reset the body clock; these are called exogenous zeitgebers e.g.

    • changes in light levels (when the day turns to evening)

    • reading by lamplight before bedtime instead of the main light

• The suprachiasmatic nuclei (SCN) in the hypothalamus detects the level of light present

  • The SCN then uses this information to coordinate the activity of the entire circadian system by either triggering sleep or telling the body to wake up

• Melatonin plays a role in triggering sleep by responding to a decrease in the levels of light

  • This explains why you may feel sleepy in a darkened cinema even when it is the middle of the day

• Melatonin secretion correlates with increased feelings of sleepiness about two hours before someone’s regular bedtime

  • It is very difficult for sleep to occur before this two-hour window

    • This explains why it sometimes feels impossible to sleep if you go to bed earlier than usual - your body is simply not ready for sleep

  • Once melatonin has been secreted there is a point at which the 'sleep gate' opens and sleep is inevitable (known as being in a hypnagogic state)

Changing circadian rhythms

• Shift work and jet lag have been found to lead to the desynchronisation of circadian rhythms and can lead to adverse cognitive and physiological effects e.g.

  • memory lapses

  • poor concentration

  • issues with fertility

  • increased risk of cancer, heart disease and diabetes

• Siffre (1973) conducted research (a case study) in which he spent two months in a cave deprived of light and sound

  • When Siffre emerged from the cave he found that his circadian rhythm remained between 24-25 hours

  • He also stated that he had kept to his usual habits i.e. going to sleep and waking at the usual times

Evaluation of circadian rhythms research

Strengths

• Subsequent research (after Siffre's original study) supports the idea that the sleep-wake cycle has a duration of just over 24 hours and that any interference with this cycle is likely to be detrimental to functioning e.g.

  • Folkard et al. (1980) implemented a 22-hour sleep-wake cycle and found that participants experienced problems with recall in their short-term memory

  • This gives the theory of the sleep-wake cycle good validity

• Research which has found that desynchronisation has deleterious effects on people has been highly useful in practice e.g.

  • Employers are advised not to allocate night shift work to employees for long periods of time to minimise the negative consequences

Limitations

• Siffre's study included one participant: himself!

  • This is a limitation as it means that

    • the results cannot be generalised to anyone else

    • he may have lost objectivity e.g. by misinterpreting his sleep-wake cycle or using confirmation bias in his reporting of the results

    • there is no one else to check his findings

• Siffre and participants in other circadian rhythm research did not exist in total darkness:

  • They had some artificial light sources throughout the procedure

  • This means that exogenous zeitgebers may have been controlling their sleep-wake cycle rather than their endogenous pacemakers

Infradian rhythms

• Infradian rhythms have less than one cycle in 24 hours e.g.

  • the menstrual cycle

    • This cycle is under hormonal control in biological females

    • It occurs roughly every 28 days

    • Rising oestrogen levels cause an egg to be released from an ovary

    • The egg travels down the oviduct to the womb

    • Rising progesterone levels thicken the lining of the womb

    • If the egg is not fertilised (by sperm cells) the lining is shed (menstruation)

  • seasonal affective disorder (SAD)

    • SAD tends to hit people during the winter months when the days are darker and longer

    • During winter months more melatonin is secreted which may result in higher levels of depression or a generalised low mood

    • Serotonin is implicated in the sleep-wake cycle (it has been linked to melatonin production) so disrupted levels of this neurotransmitter is likely to lead to imbalances in mood

• McClintock & Stern (1998) conducted research into infradian rhythms

  • Each of the 29 women in the study was given samples of pheromones from the armpits of women in the study to inhale, the samples were taken throughout their usual menstrual cycle

  • 68% of the participants found that their menstrual cycle had aligned (synchronised) with that of their pad donor

  • The researchers concluded that synching of the menstrual cycle has evolutionary advantages

    • If all the women in one tribe/clan fall pregnant at the same time there are lots of people on hand to help with childcare

• Rosen et al. (1990) conducted research into SAD

  • A large-scale survey of adults who lived in one of four different states in the USA:

    • New Hampshire

    • New York

    • Maryland

    • Florida

  • The researchers measured prevalence rates of winter SAD and summer SAD

    • They found that winter SAD rates were significantly higher the more northern the state was

    • No correlation was found between northern/southern states and summer SAD

    • The highest prevalence of winter SAD was found in age groups over 35

    • The researchers concluded that living at a more northern latitude (longer, darker winter days) is likely to lead to SAD, particularly for older adults

Evaluation of infradian rhythms research

Strengths

• McClintock & Stern's research consisted of lona gitudinal field study

  • Therefore changes over time could be tracked

  • This increases the validity of the findings (a snapshot design would not be able to show the synchronisation of the menstrual cycles)

• Rosen et al.'s large-scale survey has good reliability

  • The study used a large sample of adults from different states of the USA

  • It also used a standardised questionnaire to generate quantitative data

  • The easy replicability of the questionnaire means that the study could be repeated with different populations across the USA and, potentially, globally

Limitations

• Mc Lintock et al.’s research is potentially compromised by an array of extraneous variables e.g.

  • each woman's diet and nutrition (anything which is consumed internally could affect biological processes)

  • each woman's exercise regime (generally, fitter women may have more regular cycles)

  • pollution (any external pollutant such as car fumes could interfere with biological processes)

• Conducting a large-scale survey into SAD lacks explanatory power

  • It is not clear why SAD occurs in northern states

  • There may be other reasons for the prevalence of SAD e.g. life events

  • The extent and quality of each SAD sufferer's condition are not documented i.e. not everyone with SAD experiences it in the same way

Ultradian rhythms

• Ultradian rhythms occur more than once within a 24-hour cycle, best showcased in the cycles of sleep

• There are five recognised stages of sleep, with each cycle occurring continuously throughout the night

  • Stages 1 and 2 are known as the sleep escalator

    • At this stage of sleep, it is easy to wake the sleeper; they are in 'light sleep'

    • Slow and rhythmic alpha waves (seen via brainwaves which can be tracked using EEG) are the first sign of sleep

    • Sleep spindles occur more regularly during stage 2 sleep

    • Theta waves show that the person has gone into a deeper sleep

  • Stages 3 and 4 consist of the delta waves of deep sleep

    • Delta waves are slow and deep, with greater amplitude

    • At this stage of sleep, it is very difficult to wake the sleeper: it may take some time for them to fully wake up during stage 3/4 sleep

  • Stage 5 is REM sleep, the stage in which dreaming occurs according to research 

    • The central nervous system is 'cut off' (effectively paralysing the body) as dreaming occurs - thought to prevent people from acting out their dreams

    • REM stands for 'rapid eye movement' - the eyeballs are active during this stage of sleep, indicating that dreaming is taking place

    • Brain activity speeds up during REM sleep, with brainwaves showing similar levels of energy to the awake brain

  • Sleep stages 1-4 are categorised as NREM (non-REM) sleep; stage
    5 only is REM sleep

  • Approximately 75% to 80% of sleep time is spent in NREM

  • Dreaming can occur during NREM, but it is more common to have vivid and complex dreams during REM

• Groundbreaking research by Dement & Kleitman (1957) made the link between REM sleep and dreams

  • A sample of five participants were studied intensively

  • The participants were taken to a sleep lab

  • They had been told not to drink alcohol or caffeine before the experiment

  • An EEG machine was used to track their brainwaves during sleep

  • They were woken at specific intervals during their sleep and asked if they were dreaming and if so, what was the dream about

  • The findings showed a positive correlation between REM sleep and dreaming

  • The nature of the dreams was matched by their eye movements e.g.

    • vertical eye movements occurred with dreams of climbing ladders, watching climbers, and throwing basketballs in the air

    • horizontal eye movements occurred with dreams of watching people throw tomatoes at each other

Evaluation of ultradian rhythms research

Strengths

• Dement & Kleitman conducted a well-controlled study with extra care taken to eliminate possible confounding variables of alcohol or caffeine intake before the study

  • This means that their research is high in reliability

• Subsequent replications of their sleep study have supported Dement & Kleitman's findings

  • This gives good validity to the idea that REM sleep is associated strongly with dreaming

Limitations

• A sample of five participants means that the findings are difficult to generalise

  • This means that the findings lack external validity

  • A larger sample would also increase the robustness of the data

• Dement & Kleitman carried out their research in the 1950s which means that it may lack temporal validity

  • The use of digital technology such as smartphones, which did not exist at the time of the research, may interfere with people’s ultradian rhythms

  • This means that new research should be conducted to investigate the extent to which REM and dreaming are linked