In 2017, the Nobel Prize in Medicine was awarded to three researches for their discoveries of the molecular mechanisms controlling circadian rhythm.2 It is a an oscillating gene expression into specific proteins throughout the day andtheir degradation over night that manifest in the circadian rhythm we can observe.3
An estimated 40% of the population tend to rise earlier (“larks”) while around 30% would naturally get up later (“owls”).4 The tendencies towards morningness, normal type or eveningness are distributed in a near-Gaussian curve.
Circadian rhythm changes with age. In adolescence, it experiences a peak delay of 1-3 hours, resulting in teenagers and young adults typically being late-risers.5 Towards later life, the circadian clock advances, which is why many elderly people wake up early in the morning.6 Moreover, circadian rhythm overall dampens with age. Therefore, a rhythmic lifestyle (e.g. fixed meal times or times of light exposure) can be of great benefit for the well-being of elderly people.7
Zeitgebers: External time cues
In order not to drift away from actual daytime, your circadian clock synchronizes to different stimuli that serve as indicators for the time of day. These stimuli, scientifically called zeitgebers (after which this website is named), cause your inner clock to delay or advance, depending on the time of stimulation.4 Light is regarded to be the most important zeitgeber, but also other stimuli like food intake or physical exercise can affect your inner clock.8,9,10 Knowing how different zeitgebers affect your circadian rhythm, you can design your lifestyle in a way that your inner clock is aligned with your external schedule.
 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
 Andreani, T. S., Itoh, T. Q., Yildirim, E., Hwangbo, D. S., & Allada, R. (2015, December 1). Genetics of circadian rhythms. Sleep Medicine Clinics. W.B. Saunders. https://doi.org/10.1016/j.jsmc.2015.08.007
 Walker, M. (2017). Why we sleep: Unlocking the power of sleep and dreams. Simon and Schuster.
 Hagenauer, M. H., Perryman, J. I., Lee, T. M., & Carskadon, M. A. (2009, June). Adolescent changes in the homeostatic and circadian regulation of sleep. Developmental Neuroscience. Dev Neurosci. https://doi.org/10.1159/000216538
 Duffy, J. F., Zitting, K. M., & Chinoy, E. D. (2015, December 1). Aging and circadian rhythms. Sleep Medicine Clinics. W.B. Saunders. https://doi.org/10.1016/j.jsmc.2015.08.002
 Manoogian, E. N. C., & Panda, S. (2017, October 1). Circadian rhythms, time-restricted feeding, and healthy aging. Ageing Research Reviews. Elsevier Ireland Ltd. https://doi.org/10.1016/j.arr.2016.12.006
 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
 Lewis, P., Oster, H., Korf, H. W., Foster, R. G., & Erren, T. C. (2020, April 1). Food as a circadian time cue — evidence from human studies. Nature Reviews Endocrinology. Nature Research. https://doi.org/10.1038/s41574-020-0318-z
 Yamanaka, Y., Honma, K. I., Hashimoto, S., Takasu, N., Miyazaki, T., & Honma, S. (2006). Effects of physical exercise on human circadian rhythms. In Sleep and Biological Rhythms (Vol. 4, pp. 199–206). Springer. https://doi.org/10.1111/j.1479-8425.2006.00234.x
 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