Series 1 : Our biological clock/clocks
Since the birth of our planet, and every single day thereafter, the sun has risen in the morning and set in the evening. This clockwork-like behavior of the sun gives us our days and nights, around which we program our lives. All life forms, from the tiniest unicellular organism to plants, animals and humans, have evolved an internal biological clock that anticipates day/night cycles and helps them optimize their physiology and behavior. This internally generated daily rhythm is known as the ‘circadian rhythm’, from the Latin phrase ‘circa diem’, which translates to ‘around a day’. Lasting for around 24 hours, this continuous circular rhythm is based on our body’s reaction to the presence or absence of daylight.
This amazing biological clock sitting in the middle of your brain is called the suprachiasmatic (pronounced soo-pra-kai-as-MAT-ik ) nucleus or SCN. When our eyes are exposed to daylight, the photosensitive cells within them trigger an impulse to the SCN via our optic nerve, situated at the back of the eye. The SCN then shoots an ‘it’s daytime’ signal to the pineal gland, which secretes the cortisol hormone. Cortisol helps us feel awake and alert. This continues for as long as the sun is shining. Then as darkness comes, the SCN stops getting the light signal and relays an ‘it’s nighttime’ message to the pineal gland to prompt it to release melatonin, which acts like a natural sleeping potion and quickly induces drowsiness. This is how the circadian rhythm works, over and over, unfailingly through your lifetime.
Role of Circadian rhythms in sleep
Wakefulness and sleep are therefore under the control of the circadian rhythm…except if you are the mother of a newborn, when all bets are off! So, this rhythm coordinates a drop in core body temperature as you near typical bedtime. This temperature drop helps to initiate sleep; typically the core temperature needs to decrease by 2 to 3 degrees Fahrenheit or about 1 degree Celsius.
The graph shows our core temperature cycles over the 24-hour period. Starting at 12 pm, body temperature begins to rise, peaking late in the afternoon. The trajectory then changes; temperature begins to decline as bedtime approaches.
Do we have one master clock or multiple clocks?
Thecentral pacemaker is the SCN located in the brain and functions as the master circadian clock. Now scientists recognize that many of the body’s tissues and peripheral organs can also tell time. Thus, the circadian system resembles a clock shop rather than a single clock.
Peripheral clocks can be synchronized both by the SCN and by environmental cues, including feeding, physical activity and temperature. Peripheral clocks in different tissues control relevant physiological outputs, such as glucose production, fat storage and release of hormones. The relationship between the central and peripheral clocks, and the multiple ways by which local and external cues affect them, is an active area of research open to new discoveries.
Circadian biology and human health
Circadian clocks thus help to regulate sleep patterns, feeding behavior, hormone release, blood pressure and body temperature. Studies have indicated that chronic misalignment between our lifestyle and the rhythm dictated by our circadian clock may result in increased risk for various diseases including cancer, neurodegenerative diseases, and metabolic disorders. So, remember to listen to your body when it tells you to take a nap. It will just leave you more alert and efficient in your work!