There are people who seem to jump out of bed in the morning and there are those who struggle to wake up every day. Why is that? The short answer: circadian rhythm.

Everyone has their own circadian rhythm. People’s rhythms differ in two dimensions: One is the period in which it cycles, which in healthy adults is 24:18h +- 12 minutes. [1] The other is the time to which the clock is set which is scientifically called the ‘circadian phase’. The 3 main factors of genetics, age and environment determine the time of your clock. You can actively influence this by adjusting your environment accordingly.

Genetic disposition

In 2017, the Nobel Prize in Medicine was awarded to three researchers for their discoveries of the molecular mechanisms that control the circadian rhythm. [2] It is the oscillating gene expression in certain proteins throughout the day and their degradation overnight that manifests itself in the circadian rhythm we observe. [3]

An estimated 40% of the population tends to get up earlier (‘larks’), while about 30% would naturally get up later (‘owls’). [4] The tendencies towards morning fitness, normal type or evening fitness are distributed in an almost Gaussian curve.

Age

The circadian rhythm changes with age. In adolescence, it experiences a peak delay of 1-3 hours, which means that teenagers and young adults are typically late risers. [5] Towards later life, the circadian clock advances, which is why many older people wake up early in the morning. [6] In addition, the circadian rhythm attenuates overall with age. Therefore, a rhythmic lifestyle (e.g. fixed meal times or times of light exposure) can be of great benefit to the well-being of older people. [7]

External timers

To avoid drifting from the actual time of day, your circadian clock synchronises to various stimuli that serve as indicators of the time of day. These stimuli, scientifically called zeitgebers (after which this website is named), cause your internal clock to delay or advance depending on the timing of the stimulus. [4] Light is considered the most important zeitgeber, but other stimuli such as food intake or physical activity can also influence your internal clock. [8,9,10] If you know how different zeitgebers influence your circadian rhythm, you can organise your lifestyle so that your internal clock is in harmony with your external schedule.