It is well known that living organisms behave in different ways during the day and at night. This looks like a habit acquired by them. However, at the core of biological activities that follow the day-night pattern, there is a fascinating mechanism at work. Scientists call it ‘biological clock’.
The ups and downs of various biological activities in our bodies are together called circadian rhythm (circa= approximately, diem=day).
The day-night biological cycle or circadian rhythm has been observed in most plants and animals, even micro-organisms.
Scientifically speaking, most of the changes in biological activities arise due to a change in available sunlight. Sunlight is essential for plant photosynthesis. Light is also essential for evolved animals for seeing with their eyes. Day also has a higher temperature, which leads to a higher level of biological activity. So, the night is best spent on resting and rejuvenating tissues and organs that were injured or tired during the day.
Many human activities that change according to the time of the day are further adaptations of internal processes, responses to others’ activities, necessities of work and society, and habits.
How organisms perceive and respond to day and night?
On the earth, only humans have the knowledge of time, and it was discovered only a few millennia back. However, the biological clock that ticks like a man-made clock has evolved as a part of biological intelligence and is millions of years old. The basic premise is the same: the clock tells how much a process has moved forward; the process could be moving of a pendulum in a wall clock or slipping of sand in an hour-glass or oscillation of quartz crystal in an atomic clock or a series of biochemical reactions inside living cells.
In studies on single-celled bacteria, it has been found that just a few proteins can give rise to a nearly 24-hour cycle of chemical reactions in their bodies. Whether in bacteria or plants or animals, just a few genes do the wonder: they can switch on and off their function of producing proteins based on the stimulus provided by sunlight. This is significant, because in evolved organisms such as humans, there are billions of cells, and thus they have numerous clocks that need to synchronise among themselves.
In fruit fly (Drocera), there are specific clock genes in its cell nucleus. There also are specific clock neurons in its brain that give feedback to the body about day-night, and this feedback is used by the clock genes to control biological processes according to the diurnal cycle.
In mammals, including humans, the light signals received by eyes are sent by the brain to an endocrine gland called pineal gland. It passes on its biochemical feedback to a group of cells in the brain. This specialised group, called the suprachiasmatic nucleus, or SCN, lies in the hypothalamus portion of the brain. The biochemical signals from the pineal gland separately work on other parts of the body. SCN acts as the master clock as it coordinates the circadian rhythms of different organs. Thus, different functions and biochemical/ physiological processes in the entire body follow a delicately synchronised and sensitive circadian rhythm.
The most important biological process that occurs in humans and is synchronised with the day-night cycle is sleep, and my discussion after this point will be focussed around it.
Sleep and the circadian rhythm
In humans, a number of hormones are found closely correlated with the sleep-wake cycle, though their causation (whether the hormone causes some activities or something else causes sleep as well activity of that hormone) is not fully understood.
The pineal gland produces a hormone called melatonin, which is secreted according to the amount of light received by our eyes. When the day starts, its production goes down. Towards the evening, more melatonin is produced, and we feel sleepy. Secretion of some other hormones is, to a lesser extent, correlated with the circadian rhythm.
Please note that the hormonal activity is sensitive to light as well as sleep. In the case of sleep, a hormone could be the cause, the effect, or both.
As hormones affect different tissues and organs in different ways, the combined impact of sleep on organs is the outcome of their complex interplay. For example, when we feel sleepy, the brain starts consolidating memory, many organs including liver and heart slow down, injured tissues start their repair, the immune system gets activated in a different way than during waking, and so on.
Let us take note of some other facts relating to the sleep-wake cycle and the circadian rhythm in humans:
- Newborns do not have a circadian rhythm. But once it develops after some months, most babies follow it strictly. They must have about 9 hours of sleep at night.
- During growing years, humans need about the same amount of sleep as toddlers but the clock shifts ahead by a few hours; they remain alert till late and get sound sleep during early hours of the day.
- Adults may need slightly less sleep. However, as they grow old, the sleeping cycle advances: they tend to sleep and get up early.
- Middle-aged people are able to adjust their natural diurnal rhythm depending upon their needs of work, etc better than too young and too old people.
- Heredity, seasonal variations, tidal cycle, latitudinal differences (leading to wide differences in length of long and short days), altitude, diet and environmental factors on which one does not have control can also affect one’s circadian rhythm.
- Even some types of blind people, whose power to see is lost but eyes are intact, can transmit signals to the brain about the time of the day or presence of light!
When and how does the rhythm get disturbed?
Humans, like other evolved animals, are diurnal animals by nature. However, humans started keeping awake much beyond sunset once they discovered artificial sources of light. As civilizations advanced, night-time work and entertainment became a regular activity. Reading, and then watching television, led to a shift in sleeping patterns of the society. Now the use of smartphones for socializing and entertainment, and entertainment served by OTT and other online platforms keep people awake till late at night.
When a person does not go to bed within a few hours of sunset or is not able to get full sleep, his circadian rhythm gets disturbed. It is seen that people compensate for the duration of sleep one way or the other (e.g. by taking naps during day time, or sleeping for a longer duration in weekends) but do not give much importance to the sleep-wake timings. But both deviations disturb the circadian rhythm: not taking enough sleep leads to dysfunction of the circadian rhythm, taking sleep in instalments or taking it not according to the day-night hours leads to misalignment of biological clocks.
When we change our sleep-wake timings much away from the day-night cycle, it can cause some temporary but many permanent changes.
Staying awake till late at night, long-distance travel in which time-zones shift, consumption of alcohol or caffeine at late hours, watching screens (television, computer, mobile phone) too much and till late at night, sedentary lifestyle and sleep disorders cause mix-up in the delicate synchronization we talked about earlier.
Our sleep-wake cycle usually adapts itself to the new timings (for example, if I force myself to go to bed early and get up early for two weeks, I tend to get up at the new [early] hour effortlessly). Some people are able to adjust to biological clock misalignments without many visible problems, some suffer mildly, and some suffer badly. Even in those who do not show serious symptoms, physiological disturbances are almost certain to occur. In studies on persons doing night shifts during their entire life, it has been seen that internally almost all of them (about 97%) are not able to adjust to the night-waking regimen but remain aligned to the normal day-night cycle.
How does a disturbed circadian rhythm affect the body?
The hypothesis of this discussion is that if a person does not act the way individual biological clocks in his body are calibrated, the balance would be disturbed leading to physiological issues. The hypothesis has been proven by modern research. There is strong clinical evidence of serious long-term metabolic effects of disturbed circadian rhythm on mice and men, based on experiments in which the circadian rhythm was experimentally disrupted.
Melatonin secretion is directly linked with the sleep cycle as well as the light that our eyes receive (though there are contributing factors such as overall health and age). Its secretion below the optimal levels leads to aging, tumour growth, deposition of fat, and cardio-vascular (=relating to heart and circulation) issues such as irregular blood pressure and heart rate and platelet aggregation. Certain types of cancers are seen progressing fast when melatonin production is suppressed. Sleep-wake cycle disturbances have also been found to impact the immune system.
By inducing various undesirable physiological changes, disturbance to circadian rhythm leads to secondary diseases (many of them are therefore part of ‘lifestyle diseases’) such as hypertension, diabetes and obesity.
Studies have also found that a disturbed diurnal rhythm can lead to mental health issues, including behavioural disturbances, suicidal thoughts and depression.
Not only individual health, disturbed or deficient sleep causes problems for society also. Several studies have found that improper sleep causes direct loss of productive activity. More school dropouts, more road accidents and a rise in lifestyle diseases are among the most observed consequences of disturbed sleep documented in the research literature.
School-going children have a natural late sleep pattern, but schools open early. That keeps the children sleepy even when they prepare for school. We make them eat breakfast and force them to clean their bowels when their digestive system has not yet ‘woken up’. A Washington University study found that opening schools an hour late results in better grades as students get to sleep an additional hour in the morning.
A US study has shown that simply delaying school start by one hour (in turn, their getting one additional hour of sleep in the morning hours) could lead to an economic gain of $83 billion to the country’s economy within a decade. The earnings would come mostly from higher academic and professional performance, better adolescent health.
What can we do to keep our circadian rhythm properly tuned?
Let me share a small list of do’s and don’ts for ensuring a healthy circadian rhythm, compiled from authentic sources:
- Avoid day-time naps. If feeling drowsy in the afternoons, engage yourself in some physical activity.
This is because having a nap during the day further disturbs the circadian rhythm and in many people, it shifts the sleep time further though the body might be asking them to adjust to an ‘early to bed, early to rise’ cycle.
If you are sick or aged, or taking afternoon naps has become a habit, limit it to about 20 minutes.
- Being short of sleep during the week and binge sleeping on weekends disturb the circadian rhythm. It might compensate for the duration of sleep to a limited extent (Please note that the biological clocks are not smart enough to calculate time on a weekly basis!) but it distorts the sleep-wake cycle further.
- If you find it hard to get sleep immediately on going to bed, find out the culprit and try to remove that from your sleep-time routine. It could be anxiety, consumption of caffeine or alcohol, overeating, or an emotional issue.
- If you are forced to work late and sleep late (e.g. shift-duties, need to prepare for examinations) and that cannot be avoided, make a routine that is as near the day-night cycle as possible. If that is not practicable, keep the surroundings dark when sleeping and keep the surroundings well-lit when working.
- If you have adjusted to a new pattern and it suits you well, stick to it and do not change it now and then. If you manage the unnatural pattern without major issues but feel constantly tired or your eyes feel sleepy during waking hours, think of aligning your sleep time with the day-night cycle.
- Perform activities that need higher concentration (e.g. writing a serious article, long drive) in forenoon hours. Plan road journeys in a way that you don’t have to drive during the early morning hours.
- If you feel sleepy rather too early in the evening and you need to keep awake, think of having exposure to more light during afternoons and evenings.
- Avoid discussing matters that can cause tension just before going to bed.
- Reduce the use of screens that emit light (smartphones, computers, etc) late at night. If you have strong white-light-emitting fixtures (e.g. tube light and LED light) at home, dim them. This is because the white or bluish-white light sends the signal of sunlight to the eyes. Also note that laptop and smartphone screens emit blue light unless you change the settings for blue light reduction.
- Turn off electronic devices at least half an hour before going to bed.
- Try to reduce the ‘sleep-impairment burden’ on children by adjusting their sleep-cycle to suit their natural circadian rhythm.
- Circadian Rhythm, Lifestyle and Health: A Narrative Review
- Health Implications of Disrupted Circadian Rhythms and the Potential for Daylight as Therapy
- New Insights Into the Circadian Rhythm and Its Related Diseases
- Pathways magazine (Circadian issue)
- Sleep, circadian rhythms and health
*About the author: This article has been contributed by MK. He does not like to call himself a rationalist but insists on scrutiny of apparent myths as well as what are supposed to be immutable scientific facts. Please don’t take the views of the author as the views of Raag Delhi.