biological rhythms

biological rhythms

cyclical patterns within biological systems that have evolved in response to environmental influences, e.g. day and night.

2 key factors that govern biological rhythms:
1. endogenous pacemakers (internal) - the body’s biological clocks
2. exogenous zeitgebers (external) - changes in the environment.

circadian rhythms

24-hr cycle, aka the 'body clock', which is reset by levels of light.

e.g. the sleep-wake cycle, which dictates when humans and animals should be asleep and awake. Light provides the primary input to this system, acting as the external cue for sleeping or waking. Light is first detected by the eye, which then sends messages concerning the level of brightness to the suprachiasmatic nuclei (SCN). The SCN then uses this info to coordinate the activity of the entire circadian system. Sleeping and wakefulness aren’t determined by the circadian rhythm alone, but also by homeostasis. When an individual has been awake for a long time, homeostasis tells the body that there’s a need for sleep cuz of energy consumption. This homeostatic drive for sleep increases throughout the day, reaching its maximum in the late evening, when most people fall asleep.

Body temp is another circadian rhythm. Human body temp is at its lowest in the early hrs of the morning (36°C at 4:30am) and at its highest in the early evening (38°C at 6pm). Sleep typically occurs when the core temp starts to drop, and the body temp starts to rise towards the end of a sleep cycle promoting feelings of alertness first thing in the morning.

AO3 - strength of circadian rhythms: research support

Siffre (1975) investigated effect of light (an external cue) on circadian rhythms and 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. suggesting his 24-hr sleep-wake cycle was increased by the lack of external cues, making him believe one day was longer than it was, and leading to his thinking that fewer days had passed.

COUNTER: as the researcher and sole pp in his case study, issues with generalisability of Siffre’s conclusions.

COUNTER: Aschoff & Weber (1962) studied pps living in a bunker. The bunker had no windows and only artificial light, which the pps were free to turn on and off as they pleased. the pps settled into a longer sleep/wake cycle of between 25-27hrs. These results, along with Siffre's findings, suggest that humans use natural light (exogenous zeitgebers) to regulate a 24-hr circadian sleep-wake cycle, demonstrating the importance of light for this circadian rhythm.

AO3 - limitations of circadian rhythms

individual differences
Duffy et al. (2001) → 'morning people' prefer to rise and go to bed early (about 6 am and 10 pm) whereas 'evening people' prefer to wake and go to bed later (about 10 am and 1 am).

This demonstrates that there may be innate individual differences in circadian rhythms, which suggests that researchers should focus on these differences during investigations.

Buhr et al. (2010)
temp may be more important than light in determining circadian rhythms. Buhr et al. → fluctuations in temp set the timing of cells in the body and caused tissues and organs to become active or inactive. Buhr claimed that info about light levels is transformed into neural messages that set the body's temp. Body temp fluctuates on a 24-hr circadian rhythm and even small changes in it can send a powerful signal to our body clocks.
This shows that circadian rhythms are controlled and affected by several different factors, and suggests that a more holistic approach to research might be preferable.

endogenous pacemakers

internal mechanisms that govern biological rhythms, in particular, the circadian sleep-wake cycle. they can be altered and affected by the environment. e.g. although the circadian sleep-wake cycle will continue to function without natural cues from light, research suggests that light is required to reset the cycle every 24hrs. (Siffre and Aschoff & Weber)

the SCN is closely linked to the pineal gland, both of which are influential in maintaining the circadian sleep/wake cycle.

the SCN, in the hypothalamus, is the main endogenous pacemaker. It controls other biological rhythms, as it links to other areas of the brain responsible for sleep and arousal. The SCN also receives info about light levels (an exogenous zeitgeber) from the optic nerve, which sets the circadian rhythm so that it’s in synchronisation with the outside world, e.g. day and night.

The SCN sends signals to the pineal gland, which leads to an increase in the production of melatonin at night, helping to induce sleep. The SCN and pineal glands work together as endogenous pacemakers; but, their activity is responsive to the external cue of light.

exogenous zeitgebers

influence biological rhythms. environmental events that are responsible for resetting the biological clock of an organism. They can include social cues such as meal times and social activities, but the most important zeitgeber is light, which is responsible for resetting the body clock, keeping it on a 24-hr cycle.

The SCN contains receptors that are sensitive to light and this external cue is used to synchronise the body's internal organs and glands. Melanopsin, which is a protein in the eye, is sensitive to light and carries the signals to the SCN to set the 24-hr daily body cycle.

also, social cues, such as mealtimes, can act as zeitgebers and humans can compensate for the lack of natural light, by using social cues instead.

AO3 - strength of Endogenous Pacemakers & Exogenous Zeitgebers: research support for the importance of SCN

Morgan (1955) bred hamsters so that they had circadian rhythms of 20hrs rather than 24. SCN neurons from these abnormal hamsters were transplanted into the brains of normal hamsters, which subsequently displayed the same abnormal circadian rhythm of 20hrs, showing that the transplanted SCN had imposed its pattern onto the hamsters. This demonstrates the significance of the SCN and how endogenous pacemakers are important for biological circadian rhythms.

COUNTER: humans would respond very differently to manipulations of their biological rhythms compared to hamsters, not only cuz humans are different biologically, but also cuz of the vast differences between environmental contexts. This makes research carried out on other animals unable to explain the role of endogenous pacemakers in the biological processes of humans.

AO3 - strength of Endogenous Pacemakers & Exogenous Zeitgebers: research support for the role of melanopsin

Skene and Arendt (2007) → the majority of blind people who still have some light perception have normal circadian rhythms whereas those without any light perception show abnormal circadian rhythms.

This demonstrates the importance of exogenous zeitgebers as a biological mechanism and their impact on biological circadian rhythms.

AO3 - strength of exogenous zeitgebers: research support

when Siffre returned from an underground stay with no clocks or light, he believed the date to be a month earlier than it was.

This suggests that his 24-hr sleep-wake cycle was increased by the lack of external cues, making him believe one day was longer than it was. This highlights the impact of external factors on bodily rhythms.

AO3 - limitation of endogenous pacemakers & exogenous zeitgebers: biologically reductionist

research only considers a singular biological mechanism and fails to consider the other viewpoints.

e.g. the behaviourist approach would suggest that bodily rhythms are influenced by other people and social norms, i.e. sleep occurs when it’s dark cuz that’s the social norm and it wouldn't be socially acceptable for a person to conduct their daily routines during the night.

infradian rhythms

last longer than 24hrs and can be weekly, monthly or annually.

monthly: e.g. the female menstrual cycle, which is regulated by hormones that either promote ovulation or stimulate the uterus for fertilisation. Ovulation occurs roughly halfway through the cycle when oestrogen levels are at their highest, and usually lasts for 16-32hrs. After the ovulatory phase, progesterone levels increase in preparation for the possible implantation of an embryo in the uterus. although the usual menstrual cycle is around 28 days, there’s considerable variation, with some women experiencing a short cycle of 23 days and others experiencing longer cycles of up to 36 days.

yearly: e.g. seasonal affective disorder (SAD) → seasonal variation in mood, where some people become depressed in the winter. SAD is an infradian rhythm that is governed by a yearly cycle. melatonin, which is secreted by the pineal gland during the night, is partly responsible. The lack of light during the winter months results in a longer period of melatonin secretion, which has been linked to the depressive symptoms.

AO3 - strength of infradian rhythms: Reinberg (1967)

research suggests that the menstrual cycle is, to some extent, governed by exogenous zeitgebers.

Reinberg examined a woman who spent 3 months in a cave with only a small lamp to provide light. her menstrual cycle shortened from the usual 28 days to 25.7 days. This result suggests that the lack of light (an exogenous zeitgeber) in the cave affected her menstrual cycle, and thus this demonstrates the effect of external factors on infradian rhythms.

AO3 - strength of infradian rhythms: evidence to suggest that exogenous zeitgebers can affect infradian rhythms

Russell et al. (1980) → female menstrual cycles became synchronised with other females via odour exposure. In one study, sweat samples from one group of women were rubbed onto the upper lip of another group. Despite the fact that the two groups were separate, their menstrual cycles synchronised. This suggests that the synchronisation of menstrual cycles can be affected by pheromones, which have an effect on people nearby rather than on the person producing them.

These findings indicate that external factors must be taken into consideration when investigating infradian rhythms and that perhaps a more holistic approach should be taken, as opposed to a reductionist approach that considers only endogenous influences.

Evolutionary psychologists claim that the synchronised menstrual cycle provides an evolutionary advantage for groups of women, as the synchronisation of pregnancies means that childcare can be shared among multiple mothers who have children at the same time.

AO3 - strength of infradian rhythms: Penton-Volk et al. (1999)

infradian rhythms such as the menstrual cycle are important regulators of behaviour.

woman expressed a preference for feminised faces at the least fertile stage of their menstrual cycle, and for a more masculine face at their most fertile point.

These findings indicate that women's sexual behaviour is motivated by their infradian rhythms, highlighting the importance of studying infradian rhythms in relation to human behaviour.

AO3 - strength of infradian rhythms: evidence supports the role of melatonin in SAD

Terman (1988) → SAD more common in Northern countries where the winter nights are longer. e.g. Terman found that SAD affects roughly 10% of people living in New Hampshire and only 2% of residents in southern Florida.

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

ultradian rhythms

last fewer than 24hrs. e.g. the pattern of human sleep. The cycle starts at light sleep, progressing to deep sleep and then REM sleep, where brain waves speed up and dreaming occurs. This repeats itself about every 90mins throughout the night.

4 stages of NREM sleep before entering REM (Stage 5). Research using EEG has highlighted distinct brain waves patterns during the different stages of sleep.
1. Stages 1 and 2 are ‘light sleep’ stages. During these stages brainwave patterns become slower and more rhythmic, starting with alpha waves progress to theta waves.
2. Stages 3 and 4 are 'deep sleep' or slow wave sleep stages, where it’s difficult to wake someone up. This stage is associated with slower delta waves.
3. Stage 5 is REM (or dream) sleep. the body is paralysed (to stop the person acting out their dream) and brain activity resembles that of an awake person.

a person can experience up to 5 full cycles in a night.

AO3 - evaluations of ultradian rhythms

limitation: individual differences
Tucker et al. (2007) found significant differences between pps in terms of the duration of each stage, particularly stages 3 and 4 (just before REM sleep). This demonstrates that there may be innate individual differences in ultradian rhythms, which means it’s worth focusing on these differences during investigations into sleep cycles.
also, this study was carried out in a controlled lab setting, which meant that the differences in the sleep patterns couldn’t be attributed to situational factors, but only to biological differences between pps. While this study provides convincing support for the role of innate biological factors and ultradian rhythms, psychologists should examine other situational factors that may also play a role.

limitation: lack of ecological validity
the way in which such research is conducted may reveal little about ultradian rhythms in humans. when investigating sleep patterns, pps must be subjected to a specific level of control and be attached to monitors that measure such rhythms. This may be invasive for the pp, leading them to sleep in a way that doesn’t represent their ordinary sleep cycle.
This makes investigating ultradian rhythms, such as the sleep cycle, extremely difficult as their lack of ecological validity could lead to false conclusions being drawn.

a case study indicates the flexibility of ultradian rhythms:
Randy Gardener remained awake for 264 hrs. While he experienced numerous problems such as blurred vision and disorganised speech, he coped rather well with the massive sleep loss. After this experience, Randy slept for just 15hrs and over several nights he recovered only 25% of his lost sleep. he recovered 70% of Stage 4 sleep, 50% of his REM sleep, and very little of the other stages.
These results highlight the large degree of flexibility in terms of the different stages within the sleep cycle and the variable nature of this ultradian rhythm.

disruption of biological rhythms

people who work at night (shift work) often experience symptoms similar to jet lag. this is cuz the person's work schedule (exogenous zeitgeber) is at odds with their circadian sleep-wake cycle, which is governed by powerful biological factors (endogenous pacemakers).

people who work shift work often feel sleepy at work and suffer from insomnia at home.