Without sunlight, life on Earth would not exist. Looking at the natural world, it is clear how necessary sun rays are, not only for plants, through photosynthesis, but also for animals and human beings, as it is light that allows us to live, grow and prosper. Light is also “the tool” our body uses to adjust to the time of day and what that requires. To explain the relationship between our body and light, the neuroscientist Coline Weinzaepflen has written and illustrated the comic book Enlighten your clock: How your body tells time
. Originally created for children, the book is actually very useful for all ages, as it explains our biological mechanisms and the fundamental effects light has on all our daily lives. Sleep and mental health being just two examples. Starting from Enlighten your clock
we have created a brief glossary of keywords that are central to the relationship between light and our wellbeing.
The term “circadian” comes from the Latin, circa diem, “around the day”. Human beings, like other biological entities, follow certain time rhythms dictated by light that regulate many of our bodily activities. Over 24 hours our circadian cycle rhythmically activates certain bodily functions, from temperature to arterial pressure and from heartbeat frequency to our sleep-wake rhythm.
Melatonin is a hormone produced by the pineal gland. Descartes called it “the third eye” as it can “see” light. It is an endocrine gland, located in the brain, that plays a fundamental role in the physiology of sleep and the regulation of our sleep-wake cycle. The production and release of melatonin depend on the conditions of light exposure. So, it increases in the dark of the night and decreases during the day.
Early bird or night owl? There are those who wake up early in the morning and enjoy a slow breakfast and there are those who would rather spend the whole morning in bed, because they have stayed up late into the night working. All of us belong to a specific chronotype that indicates what time of day we are at our best from an energy point of view. While some people are most active when they have just woken up, others only get going in the late afternoon. This, too, is down to light.
Social jet lag
After a night out partying, when we have gone to bed late, our biological system becomes out of sync with the biological clock that normally regulates our body. This is called social jet lag, a term that takes its name from the sense of exhaustion we experience after long plane journeys and time zone changes. Social jet lag disturbs our daily routine, but it does not have a harmful effect on us if it does not happen too often.
In our next instalment we will analyse other keywords to understand how our body measures time, and we will see what role light plays in balancing our circadian rhythm.