What Is Circadian Rhythm?

All living being on the Earth including animals, plants etc. are tuned into the cycle of day and night. We are in total sync with nature’s rhythm of light and dark or activity and rest. We function well if our internal biological clock is adjusted to nature’s clock. The sun rises in the morning to mark the onset of the day or activity which peaks in the noon and subsequently decline in strength towards the latter half of the day. Our bodies typically behave in a similar manner. We get up in the morning to start the day, our body temperature and metabolism picks up as the day progresses and gradually decline, so are our energy levels and productivity towards the end of the day. To meet this rhythm of activity and rest, our bodies’ produce corresponding hormones called cortisol and melatonin respectively.

Circadian Rhythm

Cortisol is an arousal hormone, synthesized from cholesterol and produced in the adrenal gland. The release of cortisol is controlled by the central clock situated in the brain in the hypothalamic suprachiasmatic nucleus. Cortisol is the primary hormone responsible for the stress response. Expressed at the highest levels in the early morning, cortisol’s main function is to restore homeostasis following exposure to stress. Under normal circumstances, the human body produces a predictable day–night pattern of cortisol secretion. Normal diurnal variation in cortisol secretion produces higher cortisol levels in the daytime (for activity) and lower levels at night (for rest). Similarly, melatonin, the sleep hormone is released in the night. The daily fluctuations of the hormones contribute to the rhythm called circadian rhythm. This rhythm appears to be important in regulating and coordinating internal metabolic processes. 

Anybody travelling by air route from one continent to another in different time zone will experience a temporary disruption of circadian rhythm called jet lag which results in fatigue, disorientation, and sleep disturbance. It takes several hours or even days for the body to adjust to this sudden change but eventually body recovers and again get tuned to the local cycle.

Chronic elevation of cortisol can result in disruption of circadian rhythms.  Cortisol is a hormone that is associated with psychological and physical stress, inflammation, and sleep dysfunction in humans. High levels of cortisol can contribute to a multitude of adverse health conditions such as sleep disorders, hypertension and cardiovascular disease, stroke, impaired immune response, mood disturbances, decreased bone density, and abnormal glucose levels. Neurologic effects of chronically elevated cortisol secretion include chronic activation of the sympathetic nervous system leading to hypertension and cardiovascular disease.