As daylight-active animals, humans have evolved waking up in the morning and going to sleep in the evening1. When, in 1938, two researchers spent more than a month in an underground cave without daylight or any other external time cues, they found that their sleep-wake cycle still repeated, but with a period longer than 24 hours2. Today we know there’s a cellular process in humans and other living beings that works like an inner clock. It repeats in a near-24h period and sets the natural times of activity and rest. This inner clock is called circadian rhythm, from latin “circa” meaning “around” and “dies” meaning “day”. The circadian rhythm in the cells of our body is governed by a “master-clock” that sits in the suprachiasmatic nucleus (SCN) in the hypothalamus of our brain3.
What makes us tired or awake?
There are two different factors contributing to our daily levels of tiredness or alertness. The first one is homeostatic sleep pressure. It increases linearly with the amount of time spent awake and decreases during sleep. The circadian drive for arousal is the other factor: Our circadian rhythm regulates our level of alertness via the hormones cortisol and melatonin throughout the day. The “stress-hormone” cortisol, which activates body and mind, increases in the biological morning time (i.e. morning according to your inner clock) and decreases in the biological evening. The opposite holds true for the “sleep-hormone” melatonin, so that tiredness is induced in the biological evening and reduced in the biological morning4.
Sleep is facilitated by a high homeostatic sleep drive as well as a low circadian drive for arousal. This is called the two-process model of sleep regulation4.
What does my circadian rhythm depend on?
The circadian rhythm differs from person to person in period and phase, meaning that a free-running cycle may last 23:50h or 24:30h in different people5. Also, some people tend to naturally wake up earlier (“larks”) than others who tend to stay awake longer (“owls”)6. These tendencies are genetically scripted, but they also vary with age: In adolescence, our circadian rhythm experiences a delay, whereas it advances with increasing age. This is why elderly people tend to rise early in the morning7.
Find more details in the article “What determines my circadian rhythm”.
How can I change it?
To prevent the near-24h circadian rhythm to shift away over time from actual daytimes, it synchronizes to certain exogenous time cues. These so-called zeitgebers (from German “giving time”) phase-advance or phase-delay your circadian clock. The magnitude and direction of the shift depends on the stimulus and time of stimulation8. The dominant zeitgeber is light, but also stimuli like exercise, food intake or exogenous melatonin have been shown to impact circadian phase9,10,11,12.
This website will help you better understand your own rhythm and make you aware of the risks and challenges connected with circadian misalignments (jetlags). Most of all, it will give you scientific insights into how you can influence your circadian rhythm to help you get out of bed easily, be energetic throughout the day and fall asleep faster. Make your circadian rhythm your ally to be your best self.
 Walker, W. H., Walton, J. C., DeVries, A. C., & Nelson, R. J. (2020, December 1). Circadian rhythm disruption and mental health. Translational Psychiatry. Springer Nature. https://doi.org/10.1038/s41398-020-0694-0
 Czeisler, C. A., & Gooley, J. J. (2007). Sleep and circadian rhythms in humans. In Cold Spring Harbor Symposia on Quantitative Biology (Vol. 72, pp. 579–597). Cold Spring Harbor Laboratory Press. https://doi.org/10.1101/sqb.2007.72.064
 Walker, M. (2017). Why we sleep: Unlocking the power of sleep and dreams. Simon and Schuster.
 Borbély, A. A., & Achermann, P. (1999). Sleep Homeostasis and Models of Sleep Regulation. Journal of Biological Rhythms, 14(6), 559–570. https://doi.org/10.1177/074873099129000894
 Czeisler, C. A., Duffy, J. F., Shanahan, T. L., Brown, E. N., Mitchell, J. F., Rimmer, D. W., … Kronauer, R. E. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284(5423), 2177–2181. https://doi.org/10.1126/science.284.5423.2177
 Roenneberg, T., Kuehnle, T., Juda, M., Kantermann, T., Allebrandt, K., Gordijn, M., & Merrow, M. (2007, December 1). Epidemiology of the human circadian clock. Sleep Medicine Reviews. W.B. Saunders. https://doi.org/10.1016/j.smrv.2007.07.005
 Roenneberg, T., Kuehnle, T., Pramstaller, P. P., Ricken, J., Havel, M., Guth, A., & Merrow, M. (2004, December 29). A marker for the end of adolescence. Current Biology. Cell Press. https://doi.org/10.1016/j.cub.2004.11.039
 Roenneberg, T., & Merrow, M. (2007). Entrainment of the human circadian clock. In Cold Spring Harbor Symposia on Quantitative Biology (Vol. 72, pp. 293–299). Cold Spring Harbor Laboratory Press. https://doi.org/10.1101/sqb.2007.72.043
 Blume, C., Garbazza, C., & Spitschan, M. (2019, September 1). Effects of light on human circadian rhythms, sleep and mood. Somnologie. Dr. Dietrich Steinkopff Verlag GmbH and Co. KG. https://doi.org/10.1007/s11818-019-00215-x
 Youngstedt, S. D., Elliott, J. A., & Kripke, D. F. (2019). Human circadian phase–response curves for exercise. The Journal of Physiology, 597(8), 2253–2268. https://doi.org/10.1113/JP276943
 Kräuchi, K., Cajochen, C., Werth, E., & Wirz-Justice, A. (2002). Alteration of Internal Circadian Phase Relationships after Morning versus Evening Carbohydrate-Rich Meals in Humans. Journal of Biological Rhythms, 17(4), 364–376. https://doi.org/10.1177/074873040201700409
 Burgess, H. J., Revell, V. L., Molina, T. A., & Eastman, C. I. (2010). Human Phase Response Curves to Three Days of Daily Melatonin: 0.5 mg Versus 3.0 mg. The Journal of Clinical Endocrinology & Metabolism, 95(7), 3325–3331. https://doi.org/10.1210/jc.2009-2590