biological rhythms

What is a biological rhythm

a cyclical variation over a period of time in physical or psychological processes

examples of biological rhythms

the sleep wake cycle

the menstrual cycle

digestion

breathing

what are the three types of biological rhythm

Ultradian rhythms

Circadian rhythms

Infradian rhythms

what are ultradian rhythms

rhythms which are shorter than a day

examples of ultradian rhythms

heart rate

breathing

rhythms occurring during sleep

what are circadian rhythms

consistent cyclical variations over a period of approximately 24 hours

examples of circadian rhythms

the sleep wake cycle

what are infradian rhythms

rhythms that last longer than a day

example of an infradian rhythm

the menstrual cycle

what are endogenous factors

internal pacemakers- mechanisms within the body that govern biological rhythms

eg the SCN- Superchaismatic Nucleus in the hypothalamus

what are exogenous factors

External zeitgebers- environmental cues that influence biological rhythms

eg light, meal times, social events, work

how are circadian rhythms controlled

driven by our body clocks in all cells and synchronised by the SCN

this needs to be reset so our bodies are in synchrony with the world

what resets our biological clock

light- known as photoentrainment

light sensitive cells within the retina send messages about light levels to the SCN

the SCN uses this info to coordinate the activity of the circadian system

what is the pathway from light to impact on circadian system

light-

retina-

optic nerve-

superchiasmatic nucleus-

hypothalamus-

pineal gland

production of melatonin

what is the role of melatonin

regulates the sleep wake cycle by suppressing hormones that are excitatory to increase tiredness

AO3 for circadian rhythms

Support for free running circadian rhythms

support from animal studies

practical applications of research into circadian rhythms

studies on blind people highlight the importantance of external zietgebers

there is research support for the idea of free running circadian rhythms

Siffre studied himself by spending extended periods in isolation from external cues in a cave. He found his natural circadian rhythm to be around 25/26 hours but with periods of variation and change with age

Aschoff and Weaver studied participants in an underground ww2 bunker without any environmental or social cues. most participants showed rhythms of 24-25 hrs but dome were as long as 29hours

these studies show that the sleep wake cycle is mainly under endogenous control and exogenous zeigebers help to synchronise us with the environment

however it is important to consider the issue of individual differences and that siffre’s study was a case study so may lack generalisability

support from animal studies

Morgan bred mutant hamsters so they had a 20 hr circadian rhythm and then transplanted their SCNs into normal hamsters. These hamsters then displayed the mutant rhythm

This suggests that the SCN has a hugely important role in the circadian rhythm

however there may be issues with generalisability as this is an animal study

there are practical applications of research into circadian rhythms

Aschoff showed that individuals are able to compensate for the absence of zeitgebers eg natural light by responding to social cues

this can have practical applications. For example adjusting to jet lag by spending time outside and following social cues of the destination to reset your circadian rhythm. Also SAD can be treated using light boxes as an alternative to natural light

This shows the value of research into circadian rhythms as it has practical applications

Studies on blind people highlight the importance of external zeitgebers

Skene and Arendt studied blind people without a functioning pathway to allow transportation of melanopsin. They found that they have abnormal circadian rhythms.

This shows the importance of lights influence on circadian rhythms however this lacks ecological validity as in the real world there are other social factors which influence circadian rhythms

what is the sleep ultradian rhythm

a 90 minute cycle of sleep broken into 5 stages

what are the stages of the cycle of sleep

first 4 stages are NREM sleep and the

5th is REM sleep

what are stages 1&2 of the cycle of sleep

1&2 are light sleep characterised by change in electrical activity from beta to alpha waves

what are stages 3&4 of the cycle of sleep

slow wave sleep

characterised by slower delta waves

difficult to wake the sleeper

physiological repair work and release of growth hormone

what is stage 5 of the cycle of sleep

REM sleep characterised by fast desynchronised EEG activity resembling thr awake brain

beneficial for psychological health

how does the cycle of sleep change throughout the night

SWS period gets shorter and REM increases as the night progresses

what is the rest activity cycle

a 90 minute ‘clock’ throughout the day

Kleitman suggests we move progressively from a state of alertness to physiological fatigue every 90 mins

at the end our body runs out of resources resulting in loss of concentration tiredness and hunger

AO3 for ultradian rhythms

there are individual differences

there are issues with lab studies of sleep

understanding ultradian rhythms develops our understanding of biological rhythms overall

there are individual differences

there is an issue in studying sleep cycles as there are individual differences which makes observing patterns difficult

Tucker et al found significant differences between participants in terms of the duration of each stage- particularly 3 and 4. As this was a controlled lab study it may suggest innate differences in biological rhythms

While this difference is useful in understanding how our biological rhythms work, it can also make studying patterns difficult

There are issues with lab studies of sleep

The way sleep is researched in labs with controlled variables and being attached to monitors measuring rhythms can be invasive, meaning measurements do not reflect the ordinary sleep cycle.

This makes it difficult to investigate ultradian rhythms like the sleep cycle as lacking ecological validity may lead to incorrect conclusions

understanding ultradian rhythms develops our understanding of biological rhythms

Friedman and Fishers research shoes that the sleep cycle is part of a continum within the circadian rhythm. They found evidence of a clear 90 minute cycle of eating and drinking behaviour

This implies that the ultradian rhythms have value in longer biological rhythms

what drives the menstrual cycle

fluctuating hormone levels which regulate ovulation

normally governed by an endogenous system with the release of oestrogen and progesterone

there are variations in cycle length from 23-36 days with the average around 28 days

how could SAD be an example of an infradian rhythm

increased secretion of melatonin bu the pineal gland with increased darkness causes serotonin levels to drop which lowers mood

infradian rhythms AO3

The menstrual cycle can be influenced by exogenous cues

There is research support to suggest the influence of infradian rhythms on behaviour

evidence supports the role of melatonin in SAD

SAD may be an example of circadian rhythms

infradian rhythms can be influenced by exogenous zeitgebers

Russel found that female menstrual cycles became synchronised through odour exposure. It was found that smelling sweat pads from other women led to cycles becoming synchronised, even if they were separate

This suggests the influence of pheromones on menstrual cycles which implies that the idea of infradian cycles as only influenced by endogenous factors is reductionist

there is research support for the influence of infradian rhythms on behaviour

Penton- Volk et al found that women expressed a preference for feminised faces at the least fertile stage of their menstrual cycle and a more masculine face at their most fertile point

These findings indicate that a womans sexual behaviour is motivated by their infradian rhythms, emphasising their importance on human behaviour

research evidence supports the role of melatonin in SAD

terman found that the rate of SAD is more common in northern countries where winter nights are longer. It was found that SAD affects 10% of people in New Hampshire compared to only 2% of residents in southern Florida.

This suggests that SAD is in part affected by light resulting in increased levels of melatonin

SAD may be related to circadian rhythms

There is suggestion that SAD is due to disrupted circadian rhythms as changing seasons cause people to become tired earlier as it is darker. This change disrupts biological rhythms similarly to the effects of jet lag

thus SAD may be related more to circadian rhythms rather than infradian rhythms.