Biological Rhythms in Biopsychology

This section explores Biological Rhythms, covering circadian, infradian, and ultradian rhythms, including the difference between these rhythms. The notes also cover the effect of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle in psychology.

Biological Rhythms in Biopsychology

Biological rhythms are cyclical patterns in biological activity that repeat at regular intervals, influenced by both internal (endogenous) and external (exogenous) factors. These rhythms are critical for maintaining various physiological and behavioural functions in sync with the environment.

Types of Biological Rhythms

Biological rhythms are classified into circadian, infradian, and ultradian rhythms, based on their frequency.

Circadian Rhythms (around 24 hours)

Definition: Circadian rhythms are biological cycles that last approximately 24 hours. The most well-known example is the sleep/wake cycle.

Examples:

Sleep/Wake Cycle: This rhythm dictates periods of alertness and sleep, regulated by light exposure and the body’s internal clock.

Body Temperature: Body temperature fluctuates over a 24-hour period, peaking in the afternoon and dropping during the early hours of the morning, promoting sleep.

Research: Studies, such as Siffre’s cave study, show that circadian rhythms persist even without external cues, though they tend to shift slightly from 24 hours, demonstrating the role of internal clocks.

Infradian Rhythms (longer than 24 hours)

Definition: Infradian rhythms are cycles that last longer than 24 hours and often occur on a weekly, monthly, or seasonal basis.

Examples:

Menstrual Cycle: Lasts about 28 days, regulated by hormonal changes and sometimes influenced by external factors like pheromones from other women.

Seasonal Affective Disorder (SAD): A yearly rhythm related to changes in light exposure during winter months, affecting mood and energy levels.

Research: Studies show that women’s menstrual cycles can synchronise when they spend prolonged time together, indicating the influence of exogenous cues.

Ultradian Rhythms (less than 24 hours)

Definition: Ultradian rhythms are shorter cycles that occur more than once within a 24-hour period.

Examples:

Stages of Sleep: The sleep cycle consists of different stages (e.g., stages 1-4 and REM) that repeat approximately every 90 minutes throughout the night.

Basic Rest-Activity Cycle (BRAC): Involves 90-minute cycles of alertness and fatigue during waking hours, as proposed by Kleitman.

Research: Studies of the sleep cycle demonstrate how we move between stages of light and deep sleep, showing a natural ultradian rhythm in sleep architecture.

Endogenous Pacemakers and Exogenous Zeitgebers in the Sleep/Wake Cycle

The sleep/wake cycle is an example of a circadian rhythm regulated by both internal biological clocks (endogenous pacemakers) and external environmental cues (exogenous zeitgebers).

Endogenous Pacemakers

Endogenous pacemakers are internal body clocks that help regulate biological rhythms.

Suprachiasmatic Nucleus (SCN)

The SCN is a small bundle of nerve cells located in the hypothalamus and is considered the main endogenous pacemaker.

It receives information about light levels through the optic nerve and uses this to regulate the production of melatonin from the pineal gland.

The SCN synchronises bodily processes to the day-night cycle, promoting wakefulness during daylight and sleepiness in darkness.

Research: Studies in animals (e.g., Ralph et al. with SCN-lesioned hamsters) show that damage to the SCN disrupts circadian rhythms, indicating its central role in the sleep/wake cycle.

Pineal Gland and Melatonin

The pineal gland, located in the brain, releases melatonin in response to signals from the SCN, especially during the night.

Melatonin induces sleepiness by reducing body temperature and preparing the body for sleep. Its production decreases in daylight.

Research: Studies show that melatonin supplementation can help regulate sleep patterns in cases of jet lag, suggesting its importance in maintaining a regular sleep/wake cycle.

Exogenous Zeitgebers

Exogenous zeitgebers are external cues that influence biological rhythms, helping to align them with the environment.

Light

Light is the most powerful exogenous zeitgeber affecting the sleep/wake cycle. It resets the SCN, aligning the circadian rhythm with the external day-night cycle.

Exposure to natural light in the morning can help shift the circadian rhythm, promoting wakefulness, whereas exposure to artificial light at night (e.g., from screens) can delay sleep onset.

Research: Studies like those by Campbell and Murphy show that even light exposure to the back of the knees can shift circadian rhythms, highlighting light’s pervasive influence on the body clock.

Social Cues

Social interactions, mealtimes, and daily routines can also act as zeitgebers, particularly when travelling across time zones.

Adhering to local routines (e.g., eating and sleeping at local times) can help reset the sleep/wake cycle more quickly after jet lag.

Evaluation of Biological Rhythms and the Influence of Pacemakers and Zeitgebers

Strengths:

• Understanding biological rhythms has practical applications, such as improving shift work schedules to minimise health risks and designing treatments for seasonal affective disorder (SAD) through light therapy.
• Research on endogenous pacemakers and exogenous zeitgebers provides insight into issues such as jet lag and circadian rhythm disorders.

Limitations:

• Individual Differences: Not everyone’s circadian rhythms are exactly 24 hours; some people naturally have longer or shorter cycles, which can affect how they respond to zeitgebers.
• Animal Research: Many studies (e.g., on the SCN) have used animals, raising issues of generalisation to humans, as human behaviour may be influenced by additional cognitive and environmental factors.

Complex Interactions:

The interaction between endogenous pacemakers and exogenous zeitgebers is complex, and isolating the influence of each factor can be challenging.

Research by Miles et al. on blind individuals found that some blind people with no light perception still maintain a 24-hour rhythm due to social cues, emphasising that both internal and external factors are necessary to regulate rhythms accurately.

Key Terms to Remember

Circadian Rhythm: A 24-hour biological cycle, such as the sleep/wake cycle.

Infradian Rhythm: A biological cycle lasting longer than 24 hours, such as the menstrual cycle.

Ultradian Rhythm: A biological cycle shorter than 24 hours, like the stages of sleep.

Endogenous Pacemakers: Internal body clocks, such as the SCN, that regulate biological rhythms.

Exogenous Zeitgebers: External cues like light and social interactions that influence biological rhythms.

These notes cover the key aspects of biological rhythms, focusing on how circadian, infradian, and ultradian rhythms operate and how endogenous pacemakers and exogenous zeitgebers work together to regulate the sleep/wake cycle. Understanding these mechanisms is crucial for recognising how biological and environmental factors shape human behaviour and health.