Biological 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.

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 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.

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.

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.

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.

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.

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

Fig. 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.

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.

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.

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.

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'.