Circadian, Infradian and Ultradian Rhythms

Biological Rhythms

Biological rhythms are natural, biological events or functions of living organisms that follow repetitive patterns regulated by an internal clock. Their purpose is to respond to a period of environmental change. These include circadian rhythms, infradian rhythms, and ultradian rhythms. For example, the sleep-wake cycle is a biological rhythm.

Your body tends to do things in cycles, known as biological rhythms. The body can track the passage of time and change its internal state depending on the elapsed time. The body's internal clock monitors these processes and adheres to an unconscious, predetermined schedule. These cycles differ in how much time has elapsed between each completed cycle, which defines the nature of the rhythm.

Types of Biological Rhythms

Biological rhythms are composed of circadian, infradian, and ultradian rhythms. They differ in how long their cycles usually last. Although there are fixed parameters for how long and often a cycle occurs in a given period, the body does not always adhere perfectly to these cycles for various reasons.

Endogenous pacemakers and exogenous zeitgebers support these rhythms.

• Endogenous pacemakers are genetically determined, innate biological structures that use mechanisms in the body to monitor and adapt to biological rhythms. They are internal.
• Exogenous zeitgebers are external, environmental stimuli that act in kind with your endogenous pacemakers to trigger biological rhythms.

Although the body's pacemaker works internally, the environment influences it. Therefore, exogenous zeitgebers can affect your sleep-wake rhythm even though the body's pacemaker and your SCN monitor it.

Fig. 2 - There are different types of biological rhythms.

Circadian Rhythms

Circadian rhythms are cycles controlled by daylight that comprise 24 hours. They essentially occur once every 24 hours. The internal clock, also known as the master clock, controls circadian rhythms.

Circadian Rhythms: Examples

One of the most well-known examples of a circadian rhythm is your sleep-wake cycle. Your body is not just told when to go to sleep and wake up; it's much more complicated than that.

While awake, you use more resources to fuel your body with energy, including exercise, walking, eating, and even thinking. The more complex the mental and physical tasks, the more resources you must use to accomplish them. The digestive system also responds differently when you are awake than when asleep. When you sit down to eat a meal, the digestive system prepares for the meal in advance by secreting and producing certain proteins and hormones.

The body enters a resting state during sleep, and several functions slow down, such as breathing, heart, and metabolic rates. Your body becomes cooler at night as core temperature drops during the deepest stages of sleep.

Evaluating Circadian Rhythms

The effects of external elements on circadian rhythms are the subject of research. Siffre (1975) spent about six months in a cave studying the effects of light deprivation on his sleep-wake rhythm. By staying in a cave, he no longer had access to light, the exogenous zeitgeber important for helping the SCN induce sleep and maintain an internal body clock.

The natural sleep-wake cycle of 24 hours lengthened to 25-30 hours during his time in the cave, demonstrating the importance of light in keeping this system within its specific parameters. While the circadian rhythm is not entirely dependent on the exogenous zeitgeber, it does work with it to fine-tune it.

Individual differences exist in the study of circadian cycles.

Duffy et al. (2001) studied morning people and evening people. They assumed that morning people simply have their circadian clock set an hour or earlier than evening people, which would explain their sleep patterns.

In addition, Buhr et al. (2010) suggested that temperature may play a more critical role than light in determining circadian rhythms. They believed temperature sensitivity exists in the SCN and acts as a universal cue that influences the behaviour of autonomous cells in the body. Resetting temperature (also known as heat shocks) is an evolutionary response to synchronise internal circadian rhythms.

Our body temperature fluctuates in a 24-hour circadian rhythm, and small changes can result in strong signals to our biological clock. Buhr et al. (2010) confirm unfavourable factors influence and control circadian rhythms and suggest a holistic research approach rather than embracing a general idea.

Infradian Rhythms

Infradian rhythms are biological rhythms that occur in a cycle longer than 24 hours. They usually occur less than once daily and span weeks, months, and even years.

Interestingly, according to some studies, a woman's menstrual cycle may change depending on whether she lives with other women.

Although an internal clock controls the menstrual cycle based on one's biological state, Sabbagh and Barnard (1984) found women's cycles became somewhat synchronised when they lived together. However, why this is the case is unknown (some suggest it is related to pheromones, but that is up for debate).

Conversely, Yang and Schank (2006) examined data from 186 Chinese women living together in dormitories and found they did not synchronise their cycles during the year. They also reviewed previous literature on this phenomenon and found that synchronisation and cycle variability can have different causes, even pure coincidences and convergences that have been misinterpreted as synchronisation.

Ultradian Rhythms and Productivity

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

Ultradian rhythms are important for health, especially in the case of sleep. Disruption of the natural stages of human sleep can have serious consequences for health, the ability to concentrate and general cognitive functions.

Fig. 3 - Ultradian rhythms are cycles shorter than 24 hours.

Ultradian Rhythms: Examples

Ultradian rhythms can be objectively seen in our sleep patterns. Younger people need more sleep than older people. The sleep cycle consists of the following four phases:

1. Light sleep – NREM (non-rapid eye movement) sleep lasts about five minutes and is when you doze off. This is where you start to relax, and an EEG would show alpha waves.

2. Light sleep – NREM sleep continues, but theta waves begin. This phase usually lasts about 10 to 25 minutes. Body temperature drops, as do heart rate and respiratory rate. Small bursts of brain activity occur, usually to reduce the waking likelihood.

3. Deep sleep – NREM sleep continues, and muscle tone, heart rate, and respiratory rate relax even more. Delta waves occur here, and this stage can be referred to as slow-wave sleep (SWS). The metabolic rate is at its lowest. This phase is critical for recovery, cognitive function, and body and health restoration.

4. REM sleep – Rapid eye movements are observed in this phase, and brain activity increases abruptly. REM is rapid and shows similar EEG results to waking. The brain and eyes are active, but the body is resting and dreaming.

A cycle lasts about 90 minutes and comprises the phases mentioned above.