Many of our behaviours that seem random are, in fact, dependent on what time of the day it is. Our body can’t function normally and properly without keeping track of time. So how does the body keep track of time?
The biological clock
A scientist once lived in an underground cave for days to conduct a series of experiments. He had no clocks on him- no way of knowing what time of the day it was. Still, his body was able to keep track of time- making him sleepy and hungry as one would if they were living on the surface of the earth.
This suggests that we have an internal time-keeping mechanism- a biological clock that controls our bodily functions and influences our behaviours.
Deep inside the brain is a tiny region of specialized cells known as the suprachiasmatic nuclei (SCN). This master clock in our brains regulates the activity of biological clocks (specific protein molecules) present in nearly every organ and tissue of our bodies.1
These 24-hour cellular biological clocks are not unique to humans but are found in almost all the living things, dating back to millions of years as they’re found even in algae.2
Our biological clock is regulated by light
For most of our evolutionary history, we have relied upon the sun as our major light source. Therefore, our biological clock responds to fluctuations in the amount of light received by our eyes. The SCN is located in close proximity to the region of the brain where the optic nerves cross and hence it receives information about the amount of light in our environment. This, in turn, regulates the secretion of various hormones that influence our various behaviours.
For example, the hormone melatonin makes us sleepy as its production goes up when it’s dark outside. In ancient times, when it was dark after sunset, the melatonin production of our ancestors naturally went up and they felt like going to sleep. Today, most of us have to switch off our lights when we go to sleep, usually long after the sun has set.
When the sun rises in the morning, the presence of light inhibits melatonin production and signals the brain to switch to ‘awake’ mode.
We all know that a good night’s sleep is beneficial for us. Yet many of us don’t get it on a regular basis. One reason could be that we have extended our afternoons late into the night using artificial lights. It’s a good idea, therefore, to turn off the lights or make them dim at least an hour or so before we intend to go to sleep.
Moods, metabolism, immune system, and the biological clock
Serotonin is a neurotransmitter that improves mood when its levels in the brain rise. Serotonin levels respond to the light-dark cycle such that more light increases its levels while less light decreases it. This is why people feel good when they’re out on a sunny day and remark, “What a beautiful, sunny day!”.
Conversely, when the weather is not good (which is usually just another way of saying it’s not sunny), people don’t typically praise the weather much and may even experience a low mood.
In summer, there’s more sunlight available than in winter which is why it’s common to hear about the winter blues but not the summer blues.
The hormones responsible for hunger and metabolism also rise and fall over the course of the day. You feel most hungry in the morning, then at lunch and not so much at supper. This why your largest meal should be your breakfast, lunch should be fairly large, and dinner should be comparatively small.
Our immune system function is also dependent on our internal biological clock. For our ancestors, night time was rest time and a time when the body got an opportunity to heal itself. During the day, the body expended energy into other important tasks such as finding food, avoiding predators, etc. The chemicals driving the inflammatory response rise at night time which is why fevers are so intense during the night.3
Blood pressure and cognitive ability
Our blood pressure also fluctuates with the time of the day.4 It’s at its highest in the morning before 10 a.m. partly because our blood is most viscous at this time and partly because our blood vessels are more constricted than normal during this time.
This is the reason people are more likely to get heart attacks in the morning. It’s not a good idea to get your blood pressure up in the morning with intense exercise or even psychological stress. It’s common for people to say, “Go away! Don’t ruin my mood at this early hour” during the mornings. It’s as if people are disappointed more when they’re annoyed in the mornings than at other times of the day. It could be the mind’s way of avoiding stress or anger when the blood pressure is already up.
Schools typically schedule sports activities that involve intense physical exertion later in the day when our blood pressure drops. Also, schools typically schedule the most important classes in the mornings between 10 a.m. and 12 a.m. which is the time when our thinking ability is at its peak.5 This is also the time when exams and important office meetings are usually conducted.
After you’ve had lunch, your body diverts energy to digestion and less energy is available for your brain resulting in a decline in your cognitive ability. This is why students are bored, sleepy and unable to concentrate during the evening classes. This is also the time when you’re more likely to experience micro-sleeps resulting in a temporary loss of concentration so it’s not a good time to drive.
- Buijs, R. M., Van Eden, C. G., Goncharuk, V. D., & Kalsbeek, A. (2003). The biological clock tunes the organs of the body: timing by hormones and the autonomic nervous system. Journal of Endocrinology, 177(1), 17-26.
- University of Cambridge. (2011, January 27). Ancient body clock discovered that helps keep all living things on time. ScienceDaily. Retrieved November 9, 2018 from www.sciencedaily.com/releases/2011/01/110126131540.htm
- Labrecque, N., & Cermakian, N. (2015). Circadian clocks in the immune system. Journal of biological rhythms, 30(4), 277-290.
- Fabbian, F., Smolensky, M. H., Tiseo, R., Pala, M., Manfredini, R., & Portaluppi, F. (2013). Dipper and Non-Dipper Blood Pressure 24-Hour Patterns: Circadian Rhythm–Dependent Physiologic and Pathophysiologic Mechanisms. Chronobiology international, 30(1-2), 17-30.
- Foster, R. G., & Kreitzman, L. (2014). The rhythms of life: what your body clock means to you!. Experimental physiology, 99(4), 599-606.