DoctorSpeak: A synchronised sleep-wake cycle key to good health

Our eyes do more than help us see. They act as biological prompts, sensing and transmitting light signals through a complex network to a master timekeeper in the brain, the suprachiasmatic nucleus, which, in turn, regulates the release of melatonin, the sleep hormone, from the pineal gland. Melatonin promotes sleep and regulates sleep patterns. It synchronises our sleep-wake cycles and is increasingly believed to affect health and overall well-being. Light sensing by light-sensing proteins, the opsins, is essential for synchronising the internal clocks (circadian rhythms around the day-night cycle) of all biospheres with the external light-dark cycle. It ensures that physiological processes occur at the correct times in both plants and animals.

The light-sensitive pigment melanopsin, present in 1-2 per cent of retinal ganglion cells (ipRGC), absorbs blue light. Blue light predominates in LED lights and is emitted by nearly all digital devices/screens. When blue light is detected, the ipRGCs send a prompt to suppress melatonin production. Any kind of light, and in particular blue light, at night is like shining a bright flashlight directly into the light sensor’s ‘eyes’ at midnight, disrupting the biological clock, which affects melatonin’s production.

Low or absent melatonin levels disrupts the sleep cycle, and we stay awake at night. Darkness has the opposite effect, encouraging the release of the sleep hormone.

Earth’s orbit around the sun not only causes the seasons to change annually, its rotation around its tilted axis (23.4 degrees) creates the cycle of day (light) and night (darkness) approximately every 24 hours. All living creatures instinctively sense and respond to this light-dark (day-night) cycle and seasonal changes. Biological clocks in plants, animals and humans have been calibrated for billions of years. The clocks of all animals and plants continue to run smoothly, except ours, humans.

The advent of electric lighting in the late 19th century was a significant disruptive event in human history. Throughout the 20th century, affordable, widespread and accessible artificial light transformed our lives, making multi-shift work commonplace. Exposure to natural light has significantly reduced in favour of indoor artificial light, which, being of excessively short wavelengths, differs substantially from natural light. Digital devices in the last decade have only added to the non-visual photic insult (damage caused by excessive light exposure to the non-image-forming photoreceptive systems in the eye and brain, which regulate biological functions beyond conscious sight).

Dr Windred and colleagues from the UK conducted an extensive study, involving 13 million hours of personal light exposure to nearly 89,000 individuals over 10 years. They identified a clear link between night-time light exposure and coronary artery disease and heart failure, with women being particularly vulnerable. Kate Schweitzer last month in JAMA quoted Kristen Knutson, the Chair of a Committee on Sleep of the American Heart Association, saying, “So much about health has centred around what we do and how we do it — what we eat, how much we sleep or exercise — but when we do those things matters greatly, and they affect a host of biological processes.”

Disruption of daily rhythms in sleep, light exposure, meals, and activity increases the risk of obesity, diabetes, hypertension and heart disease, and improving the timing of these behaviours can significantly enhance health.

It generally takes two to three hours of dim light for melatonin to be released. Excessive exposure to light shifts the circadian clock and disrupts its rhythm. A chronically misaligned circadian clock or rhythm affects every system in the body, including excessive cortisol release, high blood pressure, and hormone signals that increase appetite and lead to overeating. It impacts memory, learning ability, mood swings, and raises irritability as well as suicidal tendencies. WHO has listed night work as a probable risk factor for cancer, as lack of melatonin impairs DNA repair that usually occurs during sleep.

Since Aaron Lerner discovered melatonin in the pineal gland in 1958 and the US FDA approved synthetic melatonin as a sleep aid, its use has grown significantly over the past 20-30 years, with millions worldwide taking it to improve sleep and combat jet lag.

In India, melatonin is generally regarded as a dietary supplement, available over the counter and on e-commerce platforms without a medical prescription. It must be remembered that melatonin is a circadian rhythm regulator, not a sleep aid.

An alarm bell rang worldwide last month when a study involving over one lakh people, presented by Marie-Pierre St-Onge at the American Heart Association meeting, revealed that those who used a melatonin supplement to sleep for over a year nearly doubled their risk of heart failure over the following five years, increasing from 2.7 per cent to 4.6 per cent. Although melatonin may not be directly linked to heart failure, other underlying issues could be. Consulting a physician is advisable to address any underlying conditions that may initially require melatonin.

Dimming lights and restoring darkness in the hours before bedtime, and reducing screen use at night, can help the body re-establish the natural rhythm that evolution designed for the entire biosphere. To experience these benefits firsthand, embark on a one-week challenge: no screens after 9 pm. Observe changes in your sleep quality and overall well-being, and share your results to inspire others.

— The writer is Emeritus Professor, PGI, Chandigarh

Factcheck: Human body's circadian system influences many important functions, including metabolism, sleep patterns, heart rate, blood pressure, body temperature, hormone levels and urine production. Circadian disruptions and lack of sleep have been associated with serious health problems ranging from cancer to obesity to depression. Some recent research also links circadian rhythms to aggressive behaviour.