‘Life on Earth is adapted to the rotation of our planet. …There are also indications that chronic misalignment between our lifestyle and the pace dictated by our internal timekeeper is associated with an increased risk of various diseases.’ —The Nobel Committee for Physiology or Medicine
OUR existence is a rhythm that continues to sound every 24 hours, throughout our lives. This is valid for all living beings: animals, plants and even microorganisms. It sounds a little poetic, a little mystical; In fact, this is our existence.
Our body functions are connected to an endogenous day and night clock. We follow this on autopilot mode, not on a conscious level, but much like breathing. We breathe all the time but we don’t have to think about breathing. Likewise, a route is marked that our body must follow every 24 hours (with individual variations of up to approximately four hours). It is an intrinsic autonomous daily routine called a circadian rhythm. Here you have a super summary of it.
As the darkness of the night fades and sunlight gradually appears, light enters our eyes and reaches the retina behind. Photoreceptor cells in the retina convert light into neural signals, which are then transmitted through the optic nerve to the hypothalamus, a part of the forebrain that regulates many autonomic functions, such as 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 mainly secreted by the adrenal glands located above our kidneys.
Being healthy is being synchronized with our circadian rhythm.
Cortisol is an arousal hormone that activates the body. Between 7 and 8 in the morning, our pancreas secretes maximum insulin and our body needs glucose, so it is time to have a good breakfast. Around 9 in the morning, testosterone levels rise, which conveys a feeling of well-being and increases energy, among other functions. Around 10am our hormones have prepared us to face the world and we are ready for the day. Our alertness reaches its peak at this time.
Between 2:00 p.m. and 6:00 p.m., our body temperature reaches its daily maximum, making our muscles more flexible and malleable. We have the greatest cardiovascular and skeletal muscle strength around 5 p.m. At 6 pm we have the highest level of lipids in our bloodstream. It is an anticipation of the nocturnal fasting period during sleep since lipid metabolism is slow. Around 8 pm, we have the highest level of neutrophils circulating in our body. As guardians against inflammation, these white blood cells are ready to take up position overnight in our cells to perform immune surveillance and respond to any cellular damage.
Around nine o’clock at night, the small cone-shaped pineal gland, located almost in the center of the brain between the two hemispheres, begins to slowly release the sleep hormone melatonin. We are tired, we start yawning and we are ready to fall asleep. Between 2 am and 4 am we have the deepest restful 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 a gradual accumulation of cortisol again and little by little we wake up, also thanks to sunlight, ready for another day.
Day after day, in addition to autonomous breathing, the heartbeat, the constant digestive work of our intestine, the functioning of our liver and kidneys, there is a schedule followed by our hormones that cause changes in our physiology at fixed times of the day.
The science of the circadian rhythm has been the subject of constant research by researchers. The experiments have surprised scientists who have discovered that even if a person is constantly kept in a dark environment for 24 hours and there are no external light stimuli, the body follows the times of the circadian rhythm. The same goes for plants. Most readers will be familiar with the plant called don’t touch me (Mimosa Pudica), which bends its leaves when touched. This plant has been widely studied. It normally folds its leaves at night and unfolds them in the sun. However, it would follow the same routine if kept in complete darkness for 24 hours, that is, folding and unfolding its leaves at fixed times of the 24-hour day.
This shows that these attributes of organisms are deeply rooted in our existence at the genetic level. In 1971, American chronobiologists finally located a gene in a fruit fly. They called it ‘period (per) gene’. The per gene is “the first discovered genetic determinant of behavioral rhythmicity.” Later, another gene called the “time gene” was also discovered. They also showed that if these genes are destroyed, the fruit fly loses its circadian rhythm. Jeffrey Hall, Michael Rosbash and Michael Young received the Nobel Prize in Physiology or Medicine in 2017 for this “clock-cracking” discovery.
The quote at the beginning of the column comes from a press release issued by the Nobel Committee at the time of awarding this Nobel Prize. Please read this again. Our state of health and illness has a lot to do with how aligned or misaligned we are with our circadian rhythm.
External stimuli, those from the environment (for example, light, temperature and especially those that we generate ourselves through our lifestyle) if they are not aligned with our circadian rhythm, can have a huge negative effect on our health and we become prone to developing various diseases. Lifestyle stimuli include what and when we eat; if we exercise, when and how; when we sleep and the quality of sleep; how we manage stress; the quality of our connection and social relationships and if we use any addictive substances.
Being healthy is being synchronized with our diurnal or circadian rhythm and being aware of it.
The writer is former health minister, associate professor of health systems and president of Pakistan Association of Lifestyle Medicine.
Published in Dawn, October 31, 2025
