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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 or the time of the year/position of the earth. Each of these examples is controlled by your body's biological rhythms!
Biological rhythms, also known as biorhythms, endogenous rhythms, internal rhythms or life rhythms, are natural, biological events or 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.
The sleep-wake cycle is an example of a biological clock (circadian rhythm). Repetition of intervals between the events allows the organism to harmonise with its environment. Our internal biological rhythms create physiological changes.
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.
Biological rhythms circadian, freepik.com/redgreystock
Key factors that influence biological rhythms in the human body
Two key factors determine biological rhythms:
The external body clocks, or exogenous zeitgebers, are affected by environmental changes.
The internal clocks, or endogenous pacemakers, are directly affected by internal, physiological factors (typically these are genetically determined).
You can read more about biological rhythms and bases of thinking, behaviour, and emotion in 'Biopsychology'.
A good example of an endogenous pacemaker is the suprachiasmatic nucleus (SCN).
The SCN is a structure located in the hypothalamus and acts as an internal clock for the circadian rhythm system within your body.
It 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 this information, which produces the key hormone melatonin.
When there is less light, higher levels of melatonin are produced which encourages the body to sleep. Conversely, if there is more light, less melatonin is produced which usually occurs during the hours you are 'supposed' to be awake.
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 increases. Screen time at night whilst you lay in bed has a large impact on the natural sleep-wake cycle within your body.
The circadian system essentially operates on a 24-hour cycle, in that these cycles occur 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 genetically determined mechanism of sleep within our body, we are still able to choose when to sleep and when to be awake.
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 in the absence of 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.
Overall, there are four biological rhythms within the body. These include circadian, diurnal, infradian, and ultradian 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 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.
Diurnal rhythms are circadian rhythms that have synced to occur primarily during daylight hours. 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 are biological rhythms that operate in a cycle lasting longer than a 24-hour period. Its rhythm patterns recur weekly, monthly or annually. The female menstrual cycle is a monthly infradian rhythm regulated by hormones.
Ultradian rhythms are cycles shorter/occur more than once every 24 hours. Ultradian rhythms have a shorter period and a higher frequency than circadian rhythms.
The sleeping stages change in frequency during the sleeping process, i.e. REM 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.
Insomnia disrupting sleep cycle, freepik.com/storyset
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 took the circadian rhythm into account 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.
Circadian rhythms directly affect all living organisms. They support the processes that are optimised according to different moments of the day or during a 24-hour cycle, coordinating mental and physical systems throughout the body.
Circadian rhythms are physiological and behavioural rhythms and they regulate our sleep-wake cycle.
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.Our internal biological rhythms create periodic physiological fluctuations.
The human biological rhythms are:
Sleep-wake cycle
Body temperature
Hormone secretion patterns
Blood pressure
Digestive secretions
Alertness
The biological clock includes circadian rhythms or activities synchronised with an environmental stimulus. Circadian rhythms are connected to the body and a master clock, or internal clock, that is located in the brain. It synchronises the alternation between cycles of alertness and sleepiness.
The biological clock is the internal mechanism that supports regulating our body’s timing processes.
The circadian rhythms align with our sleep and wakefulness between day and night.
It creates 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 changes in the environment 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.
The circadian rhythms become more evident when looking at the sleep-wake cycle, emphasising its importance.
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.
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, whilst others struggle immensely with early morning starts and struggle to fall asleep early at night.
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.
Two key factors determine biological rhythms. The external body clocks, or exogenous zeitgebers, are influenced by environmental changes. The internal clocks, or endogenous pacemakers, are directly affected by physiological factors. They influence the human body’s physical, mental and behavioural activities, the sleep-wake cycle and our temperature regulation.
Homeostasis is the ability to maintain 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. The homeostatic drive increases throughout the day; in the late evening, when most people need to sleep, it reaches its maximum capacity.
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.
The internal biological rhythms create physiological changes, making us feel sleepy when the sunsets. 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.
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