As daylight-active creatures, humans have evolved to wake up in the morning and go to sleep in the evening [1]. In 1938, when two researchers spent more than a month in a cave without daylight or other external cues, they realised that their sleep-wake cycle was still repeating itself, but at a rate slightly longer than 24 hours [2]. Today we know that there is a process in the cells of humans and other living beings that works like an internal clock. It repeats itself over a period of approximately 24 hours and determines the natural times for activity and rest. This internal clock is called the circadian rhythm, from the Latin ‘circa’ for ‘round’ and ‘dies’ for ‘day’. The circadian rhythm in the cells of our body is controlled by a ‘master clock’ located in the suprachiasmatic nucleus (SCN) in the hypothalamus of our brain [3].

What makes us feel awake or tired?

‍There are two different factors that influence our activity level. The first is homeostatic sleep pressure. It increases linearly with the time we are awake and decreases again during sleep. The other factor is the circadian rhythm. Throughout the day, it regulates our level of alertness via the hormones cortisol and melatonin. The ‘stress hormone’ cortisol activates the body and mind. It rises at the biological morning time (i.e. morning according to the internal clock) and falls at the biological evening time. The opposite applies to the ‘sleep hormone’ melatonin, whose rise in the evening and at night causes tiredness before it falls in the biological morning [4].

What does my circadian rhythm depend on?

The circadian rhythm differs from person to person in terms of period and phase. This means that a free-running cycle can last 23:50h or 24:30h for different people, for example [5]. In addition, some people naturally tend to wake up earlier (so-called ‘larks’), while others tend to stay awake longer (‘owls’) [6]. These tendencies are genetically determined, but they also vary with age: in youth, our circadian rhythm undergoes a delay – we tend to become owls – while as we age, it shifts forward. For this reason, older people tend to get up earlier in the morning [7].
You can find more details in our article ‘What determines my circadian rhythm’.

How can I influence this myself?

To prevent the circadian rhythm, which only lasts around 24 hours, from deviating from the actual time of day over time, it synchronises with certain external time indicators, such as (day)light. These so-called zeitgebers can set the circadian clock slightly forwards or backwards. The extent and direction of the shift depends on the type of zeitgeber and the time of stimulation [8]. The dominant zeitgeber is light, but stimuli such as physical activity, food intake or externally supplied melatonin (e.g. as a tablet) have also been shown to influence the circadian phase, i.e. the time of the internal clock [9,10,11,12].

This website will help you to better understand your own rhythm and make you aware of the risks and problems associated with large discrepancies between your internal and external clocks. Most importantly, it will provide you with scientific insights into how you can influence your circadian rhythm to get out of bed easier, stay energised throughout the day or fall asleep faster at night. Make your circadian rhythm your ally to be your best self.