Updated: Jul 1, 2021
Can’t quite manage to join the 5am club? Start blaming your genes.
Do you consider yourself a night owl? If so, you have likely received some criticism for your unconventional sleeping habits, however your tendency to snooze and rise in the later hours may not be due to imprudent decisions, but rather your genetic destiny.
What determines our sleep-wake cycle?
It comes down to a mustard seed-sized region within the hypothalamus (the part of the brain responsible for maintaining homeostasis), known as the suprachiasmatic nucleus (SCN). Sitting right above the optic chiasm (where the optic nerves meet), the SCN detects the presence of light and uses this stimulus to carry out its function as the master twenty-four-hour circadian clock in all mammals. Your individual circadian rhythm cycles routinely between alertness and sleepiness .
Figure 1: A side-on view showing the SCN - our biological clock that ensures circadian rhythms run on a 24-hour cycle. 
Although this internal tempo is present in all humans, its rises and falls vary between individuals. 40% of the population identify as morning people, 30% as night owls and the rest somewhere in between . This variance in circadian classifications, also known as chronotypes, can largely be explained by our genetics, as well as factors such as age, gender and environmental light levels. A 2019 study identified 351 genes that are associated with “morningness”, many of which were circadian locomotor output cycles kaput (CLOCK) genes. CLOCK genes encode a transcription factor which controls the “morningness” genes, therefore influencing the persistence and length of circadian rhythms .
But why the great variability in circadian genetics?
Theories suggest that the proverbial finger should be pointed at our hunter-gatherer ancestors for the array of circadian genetics among humans. Staggered sleeping patterns would have conferred a vital advantage to primitive humans living in small bands, as differences in sleep-wake cycles would allow for almost constant protection against predators . A study on the Hadza people, a modern-day hunter-gatherer tribe in Tanzania, found that the median proportion of adults in the group awake at any given time was at least 40%. Over twenty days and nights, or 200 hours, there were just eighteen minutes during which all members slept simultaneously .
Although modern society is no longer plagued by the threat of predation throughout the night, evolution hasn’t yet woken up to our relatively recent change in lifestyle.
The adverse effects of chronic circadian rhythm disruption
Most of us know from experience that a lack of sleep can reduce our work output and focus, though what may be surprising is the dire health consequences engendered by chronic circadian rhythm disruption. These include increased risk for metabolic syndrome, reproductive issues and cardiovascular dysfunction. Among the most impacted are shift workers and those who regularly experience jet lag .
A well-documented association is that of circadian disruption and impacted mental health; notably the development of depression, anxiety, bipolar disorder, and other mood disorders. Rodent studies found that disrupting the sleep-wake cycles of healthy animals produced symptoms of mental illness, while resynchronising the cycles could improve, or restore, psychiatric health .
The Early Bird Gets the Worm
The modern work scene is increasingly biased toward early start times, giving those with the “early bird” chronotype the upper hand. With Virgin Group founder Richard Branson rising at 5:45am each morning and Apple CEO Tim Cook waking no later than 3:45am, early risers have long been reputed to be more productive and successful . A study found that those who arrived earlier to work were regarded by their supervisors as more conscientious, and were more likely to receive higher performance ratings .
Unfortunately, standard work hours offer the short end of the stick to late sleepers, who perform at a suboptimal level in the mornings and are stripped of the opportunity to perform at their peak in the late afternoon and evening. Forced to wake with the early birds and unable to doze off until later in the night, it is not rare for these individuals to become chronically sleep-deprived .
Figure 2: Alertness levels experienced by early birds and night owls throughout the day are asynchronous .
Later start times could similarly be critical for the academic performance of adolescents. Many teenagers experience a sleep phase delay - their biological clock runs later than what is socially conventional. A teen made to rise at 7:30am could feel just as groggy as an adult made to rise at 5:30am, so early school start times can make it tricky for teens to stay focused throughout the school day .
Fortunately, there is hope yet for the ill-favoured chronotype - across ages. The COVID-19 pandemic has elicited a much-needed shift in traditional working arrangements, with the proportion of employers offering a flexible work policy rising from 30% prior to COVID-19 to 70% now . The past decade has also seen growing support for flexible hours in schools, with a focus on pushing back school start times to a later hour . It seems night owls are steps closer to getting the equal footing they have longed for.
So, if you consider yourself a night owl, it might be time to stop scolding yourself for hitting snooze and start accepting that we simply aren’t all wired to be morning people.
 Hastings MH, Maywood ES, Brancaccio M. Generation of circadian rhythms in the suprachiasmatic nucleus. Nature Reviews Neuroscience [Internet]. 2018 Jun 22;19(8):453–69. Available from: https://www.nature.com/articles/s41583-018-0026-z
 Bais B, Kamperman A, Van Der Zwaag M, Dieleman G, Vliet-Torij H, Bijma H, et al. Bright light therapy in pregnant women with major depressive disorder: Study protocol for a randomized, double-blind, controlled clinical trial. BMC Psychiatry. 2016;16. Figure 2, Sagittal view of the brain.
 Walker MP. Why we sleep : unlocking the power of sleep and dreams. New York, Ny: Scribner, An Imprint Of Simon & Schuster, Inc; 2018.
 Jones SE, Lane JM, Wood AR, van Hees VT, Tyrrell J, Beaumont RN, et al. Genome-wide association analyses of chronotype in 697,828 individuals provides insights into circadian rhythms. Nature Communications. 2019 Jan 29;10(1).
 Samson DR, Crittenden AN, Mabulla IA, Mabulla AZP, Nunn CL. Chronotype variation drives night-time sentinel-like behaviour in hunter–gatherers. Proceedings of the Royal Society B: Biological Sciences. 2017 Jul 12;284(1858):20170967.
 Evans JA, Davidson AJ. Chapter Ten - Health Consequences of Circadian Disruption in Humans and Animal Models [Internet]. Gillette MU, editor. Vol. 119, ScienceDirect. Academic Press; 2013 [cited 2021 Mar 31]. p. 283–323. Available from: https://www.sciencedirect.com/science/article/pii/B9780123969712000105
 Walker WH, Walton JC, DeVries AC, Nelson RJ. Circadian rhythm disruption and mental health. Translational Psychiatry. 2020 Jan 23;10(1).
 Schwantes M. Richard Branson and Tim Cook Wake Up at This Ungodly Hour (and You Should Too) [Internet]. Inc.com. 2017 [cited 2021 Mar 31]. Available from: https://www.inc.com/marcel-schwantes/richard-branson-and-tim-cook-get-up-every-day-at-t.html
 Yam KC, Fehr R, Barnes CM. Morning employees are perceived as better employees: Employees’ start times influence supervisor performance ratings. Journal of Applied Psychology. 2014 Nov;99(6):1288–99.
 Oura Ring. Example Circadian Rhythms [Internet]. Oura Ring. 2020. Available from: https://ouraring.com/blog/circadian-rhythms-bedtime/
 Rosen D. A Clinical Guide to Pediatric Sleep: Diagnosis and Management of Sleep Problems. JAMA. 2010 Apr 14;303(14):1427.
 Australian Bureau of Statistics. Characteristics of Employment, Australia, August 2020 | Australian Bureau of Statistics [Internet]. www.abs.gov.au. 2020 [cited 2021 Mar 31]. Available from: https://www.abs.gov.au/statistics/labour/earnings-and-work-hours/characteristics-employment-australia/latest-release#data-download
 Dunster, G., de la Iglesia, L., Ben-Hamo, M., Nave, C., Fleischer, J., Panda, S. and de la Iglesia, H., 2018. Sleepmore in Seattle: Later school start times are associated with more sleep and better performance in high school students. Science Advances, 4(12)