Life in circadian rhythm

‘Life on Earth is adapted to the rotation of our planet. … There are also indications that chronic misalignment between our lifestyle and the rhythm dictated by our inner timekeeper is associated with increased risk for various diseases.’ — The Nobel Committee for Physiology or Medicine

OUR existence is a rhythm which continues to play every 24 hours, throughout our lives. This is true for all living beings — animals, plants and even microorganisms. Sounds a bit poetic, a bit mystical — indeed that’s how our existence is.

Our body functions are hardwired to an endogenous day and night clock. We follow this in autopilot mode — not at the conscious level, but very much like breathing. We breathe all the time but we don’t have to think about breathing. Likewise, a route is set for our bodies to follow every 24 hours (with individual variations of up to four hours or so). It is an intrinsic autonomous daily routine called the circadian rhythm. Here is a super summary of it.

As the darkness of the night fades and sunlight gradually takes over, light enters our eyes and strikes the retina at the back. The photoreceptor cells of the retina convert the light into neural signals, which are then transmitted through the optic nerve to the hypothalamus, a part in the forebrain which regulates many autonomic functions, for example, temperature, hunger, sleep and emotions. The light signal stimulates a particular part of the hypothalamus called the suprachiasmatic nucleus, which then triggers the release of a hormone called cortisol. This is primarily secreted by the adrenal glands located on top of our kidneys.

To be healthy is to be in sync with our circadian rhythm.

Cortisol is an arousal hormone, which activates the body. Between 7am and 8am, our pancreas secretes the maximum insulin and our body needs glucose, so it’s time for a hearty breakfast. Around 9am, testosterone levels are elevated, which imparts a sense of well-being and boosts energy, among other functions. Around 10am our hormones have prepared us to face the world and we are ready for the day. Our alertness peaks at this time.

Between 2pm and 6pm, our body temperature reaches its daily high, which makes our muscles more flexible and pliable. We have the greatest cardiovascular and skeletal muscle strength at around 5pm. At 6pm, we have the highest level of lipids in our bloodstream. It is in anticipation of the fasting period at night during sleep since lipid metabolism is slow. At around 8pm, we have the highest level of neutrophils circulating in our body. As guardians against inflammation, these white blood cells are ready to take their position during the night in our cells to undertake immune surveillance and respond to any cell damage.

Around 9pm, the small cone-shaped pineal gland, situated almost at the centre of the brain between the two hemispheres, slowly starts releasing the sleep hormone melatonin. We are tired, start yawning and are ready to fall asleep. Between 2am to 4am we have the deepest restorative sleep. This is when we don’t even see dreams and are unconscious. During this phase we have our lowest body temperature in 24 hours. After deep sleep, there is again a gradual build-up of cortisol and we slowly wake up, also due to sunlight, ready for another day.

Day after day, apart from autonomous breathing, a beating heart, constant digestive work in our gut, the functioning of our liver and kidneys, there is a timetable followed by our hormones which bring changes in our physiology at fixed times of the day.

The science of the circadian rhythm has been a subject of constant inquiry by researchers. Experiments have startled scientists who find that even if a person is kept constantly in a dark environment for 24 hours and there are no external stimuli of light, the body still follows the timings of the circadian rhythm. The same is true for plants. Most readers would know about the plant called touch-me-not (Mimosa Pudica), which folds its leaves on being touched. This plant has been extensively studied. Normally, it folds its leaves at night and unfurls them in sunlight. However, it would still follow the same routine if kept in complete darkness for 24 hours, i.e., folding and unfurling its leaves at fixed times of the 24-hour day.

This shows that these attributes of organisms are deeply embedded in our existence at the genetic level. In 1971, a gene was finally located in a fruit fly by American chronobiologists. They called it a ‘period (per) gene’. The per gene is “the first discovered genetic determinant of behavioural rhythmicity”. Later, another gene called the ‘time gene’ was also discovered. They also demonstrated that if these genes are destroyed then the fruit fly loses its circadian rhythm. Jeffrey Hall, Michael Rosbash and Michael Young were given the Nobel Prize of Physiology or Medicine in 2017 for this “cracking the clock” discovery.

The quote in the beginning of the column is from a press release issued by the Nobel Committee at the time of bestowing this Nobel Prize. Please read this again. Our state of health and disease has a lot to do with how much we are aligned or misaligned with our circadian rhythm.

External stimuli, those in the environment — for instance, light, temperature and especially those we generate ourselves through our lifestyle behaviour — if not aligned with our circadian rhythm, can have a huge negative effect on our health and we become prone to developing various diseases. The lifestyle stimuli include what and when we eat; whether and when and how we exercise; when we sleep and the quality of sleep; how we manage stress; the quality of our social connectedness and relationships and whether we use any addictive substances.

To be healthy is to be in sync with our diurnal or circadian rhythm, and to be mindful about it.

The writer is former health minister, adjunct professor of health systems & president of the Pakistan Association of Lifestyle Medicine.

Published in Dawn, October 31st, 2025

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