Why do we feel sleepy at night and alert during the day? Why do some people thrive as early birds, while others prefer the quiet productivity of late nights? The answers lie in a fascinating biological process that governs our sleep-wake cycles: the circadian rhythm.
Understanding this internal clock can provide valuable insights into improving sleep quality and embracing healthier habits, so we asked our resident sleep expert, Christabel Majendie, to explain to us what the circadian rhythm is, how it affects your sleep, and the ways it adapts to your unique genetics, lifestyle, and environment.
What is the circadian rhythm? And how does it affect sleep?
All living organisms have a circadian rhythm. This is an internal clock that plays a vital role in synchronising all the biological processes within that system. In humans the circadian rhythm is set to roughly 24 hours; during the day this helps us to be alert and active, to eat and digest food, while during the night it helps us to rest, sleep and recover. All cells in the body have a molecular clock and these are synchronised by the ‘master’ clock situated in the brain.
The circadian clock helps us to know when to sleep by controlling the release of melatonin in the evening, when we are in dim lights. Then in the morning, the clock helps us to wake and melatonin is suppressed. But the timing of these events differs from person to person so people get sleepy and wake naturally at different times. These differences are strongly influenced by genetics so we all have an underlying tendency towards a certain time for sleep, which is called your chronotype. While most people are “intermediate types,” you also get people with chronotypes that are “evening types” or “morning types.”
As well as genetics, your circadian rhythm is influenced by environmental factors, the main one being light. This is because bright light is interpreted as daylight by the circadian system, even if it is artificial light and if the sun is up, your brain works to keep you awake and active. Light has the strongest influence on the circadian rhythm in the hour or two before you get sleepy and through the night, when it can suppress melatonin and push the circadian rhythm later, but also in the first few hours after waking when it can push the clock earlier. The more intense the light, the closer to the eyes and the higher the blue light content, the stronger the effect on the circadian rhythm. This is why it is advisable to not look at light emitting devices in the hour before bed, especially ones held close to your face, and dim the room lights a little.
Other influences on the clock are when you eat and when you exercise; if you do these things close to bed time, you can potentially push the circadian rhythm later, whereas if you do these things in the first hour of waking you can nudge the clock earlier. However, by far the biggest influence is light exposure.
A final point is that the circadian rhythm changes across the lifespan; young children tend to get sleepy and wake early, while in teenagers, the circadian rhythm becomes delayed so they can struggle to get to sleep at a reasonable time (this is exacerbated by being on a screen at night) and are very sleepy in the morning. In older adults (around 50+ years), the clock begins to advance so people might find they get sleepy and wake earlier than before.
While there are other systems in the body that influence sleep timing and duration, the power of the circadian rhythm is one that can’t be ignored if you want to understand your own sleep.
