Biological rhythms

What are biological rhythms

• Repeated patterns of changes in the body that are required by an internal clock

• Definition: Regular, cyclical changes in biological systems that repeat over time.

• They help the body stay in sync with the environment (like day–night changes).

• Controlled by:

  • Endogenous pacemakers = internal “biological clocks” (like the SCN in the brain)

  • Exogenous zeitgebers = external cues (like light and temperature)

core concepts

• Humans are cyclical beings, influenced by biological rhythms. These rhythms impact various physiological and behavioral processes.

• Three primary types of biological rhythms:

◦ Circadian: biological rhythm that takes about one day (24 hours to complete a cycle) - sleep, wake cycle

◦ Ultradian: Cycles lasting less than 24 hours, such as the stages of sleep and alertness patterns.

◦ Infradian: Cycles lasting longer than 24 hours, including menstruation, hibernation, and seasonal affective disorder (SAD).

Infradian rhythms

• this type of biological rhythm lasts longer than 24 hours to complete one cycle

• example - human menstrual cycle, hibernating behaviour of animals like squirrels and hedgehogs, seasonal affective disorder 

• Rhythms that last more than 24 hours. The key example of this is the menstrual cycle, which is influenced by the endocrine system. 

However, the endocrine system is not the only example. Cases of menstruation synchronisation among women living in close quarters is well documented. This synchronisation is thought to be the result of shared odour (pheromones) and light.

example - hibernation 

• state of reduced metabolic activity in animals that allows them to conserve energy and survive during periods of extreme cold or reduced food availability

• During hibernation, an animal’s body temperature drops, heart rate slows down, and breathing becomes shallow

• An animal enters a state where they are barely conscious and move very little - hibernation can last for several months, and animals rely on stored fat reserves to survive

research evidence for hibernation:

• Joien et al - on hibernation as an infradian rhythm, they found that the probability of a bear showing infradian rhythms during hibernation was 79% - indicating that the 24-hour light-dark cycle no longer impacts the bear during hibernation, suggesting that hibernation is a long-term seasonal infradian rhythm that extends beyond the typical 24-hour circadian cycle.

example - menstrual cycle

• the time from the first day of a woman’s period to the day before her next period

• in the first half of the menstrual cycle levels of the hormone oestrogen rise which causes the ovaries to develop and release an egg - lining of womb begins to thicken

• in the second half of the cycle , the levels of progesterone rise - helps to maintain

• regulated by hormones that either promote ovulation or immolate the uterus for fertilization

• ovulation occurs roughly halfway through the cycle when estrogen levels are at their highest and usually last for 16-32 hours

• after progesterone levels increase in preparation for the possible implantation of a developing embryo

Research evidence for the menstrual cycle:

• Denton - Volk et al

• found that women expressed a preference for feminized faces at the least fertile stage of their menstrual cycle and for a masculine face at their most fertile point

• findings indicate that heterosexual women’s sexual behaviours is monitored by thier infradian rhythms, highlighting the importance of studying infradian rhythms in relation to human behaviour

McClintock - investigated females who all went to the same college and who lived together in dorms

• the more time the people in the group spent together the more their menstrual cycle change to be closer together

• pheromones - being detected by other women shaping timing of when cycle starts

• an odourless chemical substance that we release into the environment that affects the behaviour of others

• next study - pheromones taken from women - worn a cotton pad in their armpit for 8 hours

• pads then rubbed on the upper lip of the other women at different stages of their menstrual cycle

• results - the menstrual cycle for many of the women came closer together - 68% of the women menstrual cycles were more synchronized with women whose pheromones were shared than they were at the start of the study - pheromones might be n exogenous zeitgeber that can influence the internal bioloical clock of the menstural cycle

Ultradian rhythms

• this type of biological rhythm lasts less than 24 hours

• Examples of ultradian cycles - feeding patterns of animals, the patterns of humans’ sleep move through different stages, repeat every 90 minutes 

• EEGs - research into sleep using EEGS - found distinct brain waves patterns - cycle of sleep - series of stages in sleep 

example - sleep cycle

• Non-REM sleep - divided into 3 stages

• n1 - lightest sleep - spend 5 minutes in n1 stage of sleep

• n2 light sleep - heart rate starts to decrease, boy temperature drops, electrical brain activity slows down

• n3 deep sleep - slow wave sleep - characterized by signals with much lower frequencies and higher amplitude known as delta waves - most difficult stage to be awakened from

• n3 - body repairs and regrows tissues and builds muscles crucial to strengthening the immune system - deep non rem sleep - consolidate memories into our long term memory

• on average, the entire cycle repeats every 90 minutes, and a person can experience up to 5 cycles a night

• The sleep cycle goes through the 4 stages of NREM sleep before entering the REM stage 5 and then repeating

• Stages 1 and 2 - are light sleep stages - brainwave patterns become slower and more rhythmic, starting with alpha waves, which progress to theta waves 

• stages 3 and 4 - ‘deep sleep’, slow wave sleep stages, difficult to wake someone up, and associated with slower delta waves

• stage 5 - REM (dream) body is paralyzed and brain activity remembers that of an awake person

research evidence - dement and Kleitman et al

aim - to demonstrate the link between brainwave patterns and dreaming

EEG used to record the brain activity during sleep 

participants woken at various times to report if they had been dreaming

findings:

• REM sleep is associated with dreaming

• NREM sleep is associated with fever dream reports - reported dreaming only 7%

• dream recall was higher when participants were woken during REM sleep - reported dreaming 80-90%

• average dream lasted approximately 20 minutes  

• identified different brain waves occurring at regular intervals during sleep using a male participant for up to 61 nights in a laboratory

• They found 2 different kinds of sleep: REM sleep and NREM

• REM cycles occurred approximately every 90 minutes during the night

• Sleep occurs in repeating 90-minute cycles that alternate between NREM and REM

•   dreaming mainly occurs in REM sleep, and eye movement patterns correspond to dream content

provides evidence by showing that sleep is not one continuous state but instead made up of repeating cycles of NREM and REM sleep

each full cycle - NREM - REM - repeat (lasts around 90 minutes, fitting the definition of an ultradian rhythm)

Brain activity by EEG shows physiological changes that mark transitions between stages - objective evidence that the body follows a biological rhythm within sleep

Circadian rhythyms

• This type of biological rhythm lasts approximately 24 hours

• Rhythms that last more than 24 hours. The key example of this is the menstrual cycle, which is influenced by the endocrine system. 

• super charismatic nucleus - located in the hypothalamus - optic nerve transmits electrical impulses from your eyes to your brain - endogenous pacemaker for the sleep-wake cycle ( internal clock sees timing when we need to sleep and when we wake)

• regulates the pineal gland - sleep

• exogenous zeitgebers - SCN ( internal clocks receive information about light through our eyes and optic nerve - SCN can even detect light when eyes are shut

• melatonin levels rise - less light - SCN stimulates the pineal gland to produce more melatonin to help us fall asleep

• melatonin levels fall - more light - SCN stops the pineal gland from producing melatonin to help us feel awake

research into circadian rhythms - De Corley et al - chipmunks

• control conditions - 20 normal chipmunks - SCN intact

• radio collar on each chipmunk

• released back into their natural habitat, observed for 80 days

• Research suggested that damage to SCN affected the sleep/wake cycle

• chipmunks - didn’t know when to sleep - restless during the night and vulnerable to predators

• Research indicates the importance of SCN in regulating the circadian rhythm ( sleep/ wake cycle)

example - body temperature

• human body temperature is at its lowest in the early hours of the morning (36 degrees at 4:30 am)

• Body temperature is highest in the early evening

• sleep occurs when the core temperature starts to drop and the body temperature starts to rise towards the end of a sleep cycle, promoting feelings of alertness

• first thing in the morning - influenced by several factors:

• muscular activity, digestion, and heat loss and production

research evidence - Buhr et al

• aim - to investigate whether body temperature can act a a cue (zeitgeber) for regulating circadian rhythms

• findings:

• Buhr et al - changes in body temperature can reset circadian rhythms in cells and tissues

• even small fluctuations in temperature can send powerful timing signals to the body’s internal clocks

• This shows that body temperature rhythms not only follow the circadian rhythms but also help maintain and synchronize them across the body

• Conclusion - Body temperature is both a result and a regulator of circadian rhythms

• supports the idea that biological rhythms are maintained internally, but can be influenced by physiological cues like temperature

example - sleep - wake cycle

• An approximate 24-hour cycle that determines our pattern of wakefulness and sleepiness, the circadian rhythm also dips and rises at certain times of the day, so our strongest sleep drive usually occurs in 2 dips ( between 2-4 am and 1-3 pm)

• The sleepiness we feel during these circadian dips is less intense if we have had sufficient sleep and more intense when we are sleep-deprived

• the sleep/wake cycle is influenced by both internal and external factors known as endogenous pacemakers and exogenous pacemakers.

• Disruption of Circadian Rhythms: Studies demonstrate a multitude of negative health consequences associated with disrupted circadian rhythms, including cardiovascular diseases, gastrointestinal issues, loss of bone mineral density, endocrine disruption, cancers, and metabolic syndromes.

◦ Occupations requiring disrupted sleep schedules, such as healthcare workers, submariners, and lorry drivers, are particularly susceptible.

◦ Guo et al. (2020) found that submariners, working on a 6-hour on/12-hour off schedule with limited exposure to natural zeitgebers, experience severe circadian misalignment and decreased cognitive performance.

Endogenous pacemakers

• internal mechanisms that govern and influence the patterns of our biological rhythms - there may be genetic mechanisms

• helps maintain regular rhythms in the absence of zeitgebers, but they are not perfect and need a zeitgeber to synchronize the rhythms to our individual behaviour

• The SCN is regulated by external factors, such as light and noise.

• You may find it more difficult to sleep earlier during the summer as your SCN will not release melatonin until later due to the remaining light outside. 

• SCN’s are in human and animal species and are usually in the central brain area, suggesting we have had this pacemaker throughout our evolution.

Key Example – The Suprachiasmatic Nucleus (SCN)

• The SCN is a small cluster of nerve cells located in the hypothalamus (just above the optic chiasm).

• It’s considered the main biological clock.

• The SCN receives information about light from the retina and adjusts our biological rhythms accordingly.

• Even when isolated from external light cues, the SCN maintains a roughly 24-hour circadian rhythm, although it may drift slightly.

How It Works:

1. Light detected by the retina → signal sent via optic nerve to SCN.

2. SCN sends signals to the pineal gland.

3. The pineal gland releases melatonin (a hormone that induces sleep) during darkness.

4. As light increases, melatonin secretion decreases → you wake up.

examples include:

• pineal gland

• super charismatic nucleus

the role of endogenous pacemakers in controlling the sleep-wake cycle:

• Our internal body clock is found in the hypothalamus and is called the SCN - this synchronizes our sleep-wake circadian rhythm

• The SCN receives light through the eyes (optic nerve) - when light levels drop this information is received by the SCN, causing it to fire impulses to the pineal gland, which then secretes melatonin, which causes sleepiness. This is because melatonin acts as an aid to decrease brain activity

• when light increases (daytime), melatonin levels fall, making us more alert

• without light as a zeitgeber, the process ‘free runs’ to an average of ,24 hours

• Aim:
  To investigate how endogenous pacemakers control the sleep–wake cycle without external cues (exogenous zeitgebers) such as light or clocks.

  Procedure:

  • Michel Siffre, a French cave explorer, spent several months in a cave (first in 1962, then again in 1975).

  • There was no natural light, no clocks, no radio, and no social contact — only artificial light he could turn on and off.

  • He recorded when he slept and woke to see how his body’s natural rhythm would change.

  Findings:

  • His sleep–wake cycle continued but became slightly longer than 24 hours (about 25–30 hours).

  • He still fell into a regular pattern, showing that his endogenous pacemaker (internal body clock) was regulating his rhythm even without environmental cues.

  • However, because the cycle was longer than normal, it showed that external cues (light, social cues) normally help entrain (reset) the internal clock to exactly 24 hours.

  Conclusion:
  Siffre’s study shows that:

  • The sleep–wake cycle is controlled by an endogenous pacemaker, but

  • It needs exogenous zeitgebers (like light) to stay in sync with the 24-hour day

evaluation for endogenous pacemakers

• 🧠 1. Research Support – Siffre

  P: Research by Siffre provides strong support for the role of endogenous pacemakers in controlling biological rhythms.
  E: Siffre spent several months living in a cave with no external cues such as natural light or clocks.
  E: His sleep–wake cycle continued but became slightly longer than 24 hours, showing that the body’s internal clock (endogenous pacemaker) regulates sleep and wakefulness even without environmental cues.
  L: This suggests that biological rhythms are largely driven by internal mechanisms, supporting the importance of endogenous pacemakers like the suprachiasmatic nucleus (SCN).

• 🔬 2. Methodological Flaws – Case Study

  P: However, Siffre’s study has methodological weaknesses because it was a case study.
  E: It involved only one participant — Siffre himself — which means the findings cannot be generalised to everyone.
  E: Individual differences such as age, lifestyle, or genetics may affect circadian rhythms, so the results may not apply to others.
  L: Therefore, although Siffre’s research supports the role of endogenous pacemakers, the limited sample reduces the reliability and population validity of the findings.

Applications – Jet Lag and Shift Work

P: Understanding endogenous pacemakers has important real-world applications, particularly for managing jet lag and shift work.
E: Knowing that the sleep–wake cycle is controlled by internal clocks helps scientists develop strategies to resynchronise these rhythms after disruption.
E: For example, exposure to bright light at specific times can help reset the body’s internal clock after travelling across time zones or working night shifts.
L: This shows that research into endogenous pacemakers not only improves psychological understanding but also has practical benefits for improving health and productivity.

exogenous pacemakers

• These are those external influences that affect the SCN. They are cues from the environment which help us regulate our timings for sleep. 

• These can become confused if an individual crosses time zones and their SCN does not match up with their external environment. This is known as jet lag and can take a few days for the two influences to become aligned once more.

• Exogenous pacemakers are external environmental cues that influence or regulate biological rhythms.
  They help entrain (reset or synchronize) our internal body clocks — the endogenous pacemakers — to match the external world.

example - light

• the most important to reset the body clock each day, keeping it on a 24-hour cycle - key zeitgeber in humans, it can reset the body’s main endogenous pacemaker - the SCN, and thus plays a role in the maintenance of the sleep-wake cycle

• light has an impact upon melatonin production and therefore sleep /wakefulness - light also has an indirect influence on key processes in the body that control such functions as hormone circulation and blood circulation.

role of exogenous zeitgebers in controlling the sleep-wake cycle - can use for circadian rhythm too

• Michel Siffre

• spent 2 months underground in caves without sunlight and clocks

• monitored bodies’ activities and the number of things, including when he slept

• Despite the absence of natural daylight, he maintained a regular sleep /wake cycle of around 25 hours

• woke up and went to sleep at regular fixed times

• lost track of time

• Conclusions - we have an internal biological clock that controls our circadian rhythms - controls when we sleep and wake up

• appears that our sleep-wake cycle is nearer to 25 hours rather than 24

• shows the need we have for exogenous zeitgebers such as light to regulate our 24-hour sleep/wake cycle

Practical applications of understanding circadian rhythms:

• the treatment of illnesses

• chronotherapeutic ( study of how timing affects drug treatment ) - optimal time to deliver and other interventions

• Medication that helps fight the illness and limits the harm to the patient highlights the importance of understanding circadian rhythms and their impact on human life.

evaluations for exogenous zeitgebers

 strengths:

P: Research by Vetter et al. supports the role of exogenous zeitgebers, particularly light, in regulating circadian rhythms.
E: They studied office workers who were exposed to either natural light or artificial light conditions.
E: Participants working under natural light maintained synchronization between their sleep–wake cycle and the natural light–dark cycle, while those under artificial light drifted out of sync.
L: This shows that light acts as a key exogenous zeitgeber, helping to entrain biological rhythms to the external environment.

Applications – Shift Work and Jet Lag

P: Research into exogenous zeitgebers has useful real-world applications in managing shift work and jet lag.
E: Understanding how light entrains the body clock has led to interventions like strategic light exposure and light therapy to help workers and travellers adapt their circadian rhythms.
E: For example, exposure to bright light during night shifts can help workers stay alert and align their rhythms to new schedules.
L: This demonstrates the practical value of understanding exogenous zeitgebers in improving wellbeing and productivity.

weakness:

Contradictory Research – Luce and Segal (1966)

P: However, Luce and Segal’s findings challenge the idea that light alone controls biological rhythms.
E: They studied people living in the Arctic Circle, where daylight can last up to 24 hours in summer or be absent in winter.
E: Despite these extreme light conditions, residents still maintained regular sleep patterns, suggesting other factors (e.g. social cues or internal clocks) also regulate rhythms.
L: This implies that exogenous zeitgebers like light are not the sole influence — they interact with endogenous pacemakers and social routines to maintain stability.

Theoretical Flaws – Individual Differences

P: A limitation of research on exogenous zeitgebers is that it often ignores individual differences in how people respond to environmental cues.
E: Some people, known as “morning types” (larks), are naturally more alert earlier in the day, while “evening types” (owls) prefer later activity.
E: This suggests that the effect of light and other zeitgebers varies between individuals due to genetic or lifestyle factors.
L: Therefore, the theory that exogenous zeitgebers universally entrain biological rhythms is overly simplistic and may not apply equally to everyone.

comparing biological rhythms

• infradian lasts - more than 24 hours

• bodily processes - menstrual cycle, seasonal affective disorder, hibernation, and migration

• frequency of each rhythm - less than once per 24 hours, once every month or season

• circadian rhythm - about 24 hours

• bodily processes - sleep-wake cycle, body temperature, melatonin release cortisol levels

• frequency of each rhythm -once every 24 hours

• ultradian rhythm - less than 24 hours

• stages of sleep, REM, heartbeat, and digestion

• more than once every 24 hours (90 mins)

disrupting biological rhythms

• the circadian rhythm is intolerant of any major alterations in sleep and wake schedules, causing the biological clock to become completely out of balance

• Jet lag - we experience it after crossing into a different time zone

• winter et al - calculated that it takes one day to adjust to each hour of time change

• 2 adjustments our body clocks can make when crossing time zones - phase advance and phase delay

• phase delay - traveling east to west extends your day

• phase advance - traveling west to east shortens the day

• shift lag - negative effects on workers caused by rotating shifts

• psychological effects - mood disorders such as depression and anxiety, cognitive issues such as poor memory and attention

• physiological effects - fatigue, sleep disturbance like insomnia , increased risk of chronic disorder

• Knutsson et al - found that individuals who worked shifts for more than 15 years were 3 times more likely to develop heart disease than non-shift worker - One main effect is the difficulty in sleeping during the day for night shift workers