Biological Rhythms

Have you ever wondered how your body knows when to go to sleep and when to be awake? Have you noticed that you feel more alert at certain times of the day than others? Maybe you have even noticed that you feel different depending on the weather, the time of the year, and the earth's position. Your body is in tune with the environment. Each of these examples is controlled by your body's biological rhythms. So, what are biological rhythms?

Biological Rhythms Biological Rhythms

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Table of contents
    • We are going to explore biological rhythms in psychology.
    • First, we will provide a biological rhythms definition.
    • We will then explore the four types of biological rhythms: circadian rhythms, diurnal rhythms, infradian rhythms, and ultradian rhythms.
    • Throughout our discussion on human biological rhythms, we will highlight those of particular importance, namely the biological clock, also known as the circadian rhythm.

    Biological Rhythms, city underneath a large clock showing the time, StudySmarterFig. 1 - Biological rhythms are biological internal rhythms.

    Biological Rhythms: Definition

    Biological rhythms are biological internal functions occurring in living organisms. Biological rhythms follow repetitive patterns determined by internal and environmental changes. Human biological rhythms include the sleep-wake cycle and the menstruation cycle, and they create physiological changes.

    Biological rhythms allow the organism to harmonise with its environment.

    Human bodies react to the earth's rotations in relation to the sun and the moon, making us feel sleepy when the sun goes down. This alternation between day and night creates human biological rhythm changes.

    Two key factors determine biological rhythms:

    • The external body clocks, known as exogenous zeitgebers, are affected by environmental changes.

    • The internal clocks, known as endogenous pacemakers, are directly affected by internal physiological factors (typically, these are genetically determined).

    Biological Clock: Circadian Rhythm

    A biological clock is an internal clock. The suprachiasmatic nucleus (SCN) is the human biological clock and is an example of an endogenous pacemaker.

    The SCN is a structure located in the hypothalamus and acts as an internal clock for the circadian rhythm system within your body.

    The SCN helps maintain the sleep-wake cycle by detecting changes of light in your environment based on photosensitive cells within the retina of your eyes. The SCN innervates (stimulates through the nerves connected) the pineal gland based on the light information, which produces the key hormone melatonin.

    When there is less light, higher melatonin levels are produced, which encourages the body to sleep. Conversely, if there is more light, less melatonin is produced, usually, during the hours you are 'supposed' to be awake.

    Biological Rhythms, diagram of the suprachiasmatic nucleus and other brain areas involved in detecting light and sleep-wake cycle, StudySmarter Fig. 2 - The suprachiasmatic nucleus (SCN) is the brain’s clock mechanism. The clock sets itself with light information received through projections from the retina¹.

    A lot of medical researchers within the last few years have highlighted some concerning issues with the encroaching problem of increased screen time and light exposure modern society faces. The number of people spending more time looking at a screen has increased drastically.

    Screen time at night whilst you lay in bed greatly impacts the natural sleep-wake cycle within your body.

    The circadian system operates on a 24-hour cycle (it occurs once every 24 hours). It typically depends on the feedback given by your body on how long you have been awake, which relies on the body's homeostasis mechanisms.

    If you are awake for too long, the sleep-wake cycle will detect this and signal the need for sleep, and this reservoir of a rising need to sleep builds up over the course of the day until it reaches its maximum capacity.

    Despite our endogenous pacemakers maintaining the determining mechanism of sleep within our body, we can still choose when to sleep and when to be awake (to an extent), although the longer you go without sleep, the more likely you are to experience concerning symptoms such as hallucinations and paranoia.

    Exogenous zeitgebers work in collaboration with your endogenous pacemakers. They use outside, external factors as a prompt to trigger different biological rhythms within your body, including the one for sleep.

    Siffre (1975) spent isolated time in a cave, where there was no ability to keep the time nor the ability to rely on natural light to estimate the time, either. He wanted to understand how his sleep-wake cycle was affected without these time-keeping devices.

    Interestingly, Siffre's sleep-wake cycle changed to a 25-30 hour cycle instead of the 24-hour one. This implies that whilst the circadian rhythm is mostly able to establish a 24-hour cycle; it works in tandem with exogenous zeitgebers to maintain and fine-tune it.

    Four Types of Biological Rhythms

    There are four biological rhythms within the body: the circadian, diurnal, infradian, and ultradian rhythms. They differ in duration and occurrence.

    Circadian Rhythms

    As we established above, circadian rhythms are a biological version of a clock inside humans, animals, plants and possibly almost all living cells, and they occur once every 24 hours. These rhythms primarily rely on light to operate and keep the 24-hour cycle.

    Daylight regulates the clock cycles comprising one day and one night.

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

    Biological rhythms, upside down shot of a woman sleeping with her dog on a fur rug, StudySmarterFig. 3 - The sleep-wake cycle is a famous example of a circadian rhythm.

    Diurnal Rhythms

    Diurnal rhythms are circadian rhythms that have synced to day and night. The circadian rhythm period may vary beyond 24 hours. When the rhythm is synchronised with the day and night cycle, it is called a diurnal rhythm.

    Body temperature and heart rate vary throughout the day in sync with the natural day and night cycle.

    Infradian Rhythms

    Infradian rhythms are biological rhythms that operate in a cycle lasting longer than a 24-hour period. Its rhythm patterns occur weekly, monthly or even annually. The female menstrual cycle is a monthly infradian rhythm regulated by hormones.

    Ultradian Rhythms

    Ultradian rhythms are cycles that are shorter or occur more than once every 24 hours. Ultradian rhythms have a shorter period and a higher frequency than circadian rhythms.

    The sleeping stages (not to be confused with the sleep-wake cycle) change in frequency during the sleeping process, i.e. REM (rapid eye movement) sleep, deep sleep, and light sleep.

    Having a regular cycle of these stages is incredibly important for health in general, and disruption can cause various issues.

    Disruption of biological rhythms can cause a host of issues, including slowing reaction times, affecting our cognitive skills, problem-solving skills, and ability to focus on things.

    Cziesler et al. (1982) found that those with sleep patterns affected by their work situations, such as working day and night shifts without time to adjust properly, had health and sleep problems.

    Cziesler et al. (1982) suggested that if a working pattern that considered the circadian rhythm was established, it would improve health and sleep. They implemented something called phase delay and a 21-day shift pattern. This resulted in increased productivity and job satisfaction.

    More on Circadian Rhythms

    Circadian rhythms directly affect all living organisms. They support optimised processes according to different moments of the day or during a 24-hour cycle, coordinating mental and physical systems throughout the body.

    They respond to natural cycles of light and dark and reset the organism via the levels of light received. When the retina perceives light it triggers a response, alternating the levels of hormones excreted or body temperature within specific periods for optimum energy expenditure.

    However, that's not to say all living organisms are bound to a circadian rhythm. Rather, they have a biological rhythm that differs depending on the goal they are trying to achieve.

    The Biological Clock and the Circadian Rhythm

    The biological clock is the internal mechanism that regulates our body’s timing processes. Circadian rhythms are connected to the body and a master clock, or internal clock, that is located in the brain.

    Circadian rhythms create a stable and harmonious cycle of rest that allows us to have more energy to be active during the day. This biological circadian system helps humans adapt to environmental changes and allows us to anticipate changes in our environment like radiation, temperature and food opportunities.

    The biological clocks affect the circadian rhythms, but not all biological clocks are circadian.

    Plants, for example, adjust themselves to changes related to the seasons or the biological clock. However, the process lasts beyond the 24-hour cycle, which is longer than the circadian cycle.

    Human Biological Rhythms

    Humans are complex creatures and require different monitoring levels to achieve healthy functioning. The human biological rhythms are:

    • Sleep-wake cycle

    • Body temperature

    • Hormone secretion patterns

    • Blood pressure

    • Digestive secretions

    • Alertness

    Issues with Studying Biological Rhythms

    It is hard to truly study the biological rhythms in that studying animals has generalisability issues, and studies removing natural light still allowed for artificial light, reducing the validity of their findings. There are also issues with individual differences, in that people can be 'morning people' and 'evening people'.

    Some find it easy to wake up in the morning and have an average of six hours of sleep, while others struggle immensely with early morning starts and fall asleep early at night.

    Biological Rhythms - Key takeaways

    • Biological rhythms are natural, biological events or functions occurring in living organisms that follow repetitive patterns determined by internal factors and environmental changes.
    • The suprachiasmatic nucleus (SCN) is the human biological clock and is an example of an endogenous pacemaker.
    • The four biological rhythms are circadian, infradian, diurnal, and ultradian.
    • Circadian rhythms are biological rhythms that last 24 hours and are commonly known as the 'biological clock', which is reset by levels of light. Diurnal rhythms are circadian rhythms that have synced to day and night.
    • Infradian rhythms are biological rhythms that operate in a cycle lasting longer than a 24-hour period (menstrual cycle). Ultradian rhythms are cycles that are shorter or occur more than once every 24 hours (sleep stages).


    1. Fig. 2: Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020). 4.1 What Is Consciousness? In Psychology 2e. OpenStax. via Creative Commons LicenseThis work is licensed under a Creative Commons Attribution 4.0 International License.
    Frequently Asked Questions about Biological Rhythms

    What are biological rhythms?

    Biological rhythms are natural, biological events or functions that occur in living organisms. Biological rhythms follow repetitive patterns responding to a period of environmental changes by keeping repetitive intervals between events. This allows the organism to harmonise within its environment. Our internal biological rhythms create physiological changes.

    How do our biological rhythms influence our daily functioning?

    Biological rhythms ensure we function optimally during the day, for instance, by ensuring we get enough sleep by maintaining a sleep-wake cycle, and ensuring our reproductive functions occur properly, for instance, in the menstrual cycle, which can affect our daily functioning.

    What is the relationship between biological rhythms and homeostasis?

    Homeostasis maintains the internal bodily environment in constant equilibrium through dynamic interactions in physiological and biochemical systems. The homeostasis mechanism and the circadian rhythm, which is one of the biological rhythms, collaborate to determine sleep-wake cycles, amongst other vital functions. This ensures our various bodily functions, such as hormone release, occur at the right times.  

    What is the biological clock circadian rhythm?

    A biological clock is an internal clock. The suprachiasmatic nucleus (SCN) is the human biological clock and is an example of an endogenous pacemaker. The circadian rhythm is an effect of a biological clock. Not all biological clocks follow a circadian rhythm, which has a 24-hour cycle. It works by regulating the alternation between cycles of alertness and sleepiness.

    What is the biological rhythm of our sleep?

    The internal biological rhythms create physiological changes, making us feel sleepy when the sun sets. When people are awake for a long time, the homeostatic physiology of the sleep-wake cycle sends negative feedback to the body to signal the need for sleeping to reset the energy levels. 

    Test your knowledge with multiple choice flashcards

    Exogenous zeitgebers are innate biological structures.

    Circadian rhythms developed to help humans adapt to changes in the environment.

    The body's pacemaker can be influenced by the environment.

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