By Jenna Pfeifer
What enables some species to regularly rise with the sun and rest with the stars? The biological mechanism for this is believed to be an evolutionary adaptation to the earth’s 24 hour rotation1. It is defined as circadian rhythm and found universally in organisms from cyanobacteria to plants and fruit flies. You could be reading this at 2 a.m. and disagreeing; after all, you’ve clearly proven me wrong just by being awake at this early hour. But the very same technology you are using to read this article has co-opted our brains into believing we have more control over our circadian rhythms than we truly do.
Shedding light on circadian health
Satchin Panda, the leading chrono-biologist of the Salk Institute in California, has coined the term digital jetlag to describe the phenomenon of our body and mind existing in separate time zones and is investigating the harmful effects of this desynchronization on our health. Artificial light can shift the timing of circadian clocks and prohibit sleep. Increased light exposure at night has been linked to reproductive issues, immunological disease, and various psychiatric disorders2,3. Depression and suicide rates within populations have been shown to increase as you go from the equator to higher latitudes, and this increase is seasonal with more depression in the winter. Indeed, bright-light therapy is an approved treatment for seasonal affective disorder and has also been shown to have beneficial effects in adults with non-seasonal Major depressive disorder4.
There is still a lot to learn about the ways in which light can influence our health, but this field of research has exploded over the past decade. Issues concerning circadian biology are now actively debated in society. For example; whether schools should modify start-times to synchronize schedules to student’s daily rhythms5, the impacts of light pollution, and if daylight savings should be eliminated. Furthermore, If we understand the cross-talk between circadian biology and the immune system this might indicate windows of time where infection risk is increased and thus help to regulate future pandemics6.
The biology underlying circadian rhythms
Albert Einstein said that the only reason for time is so that everything doesn’t happen at once. In humans, each organ has its own circadian rhythm which regulates our physiological activity, depending on the time of day, so that all biological activity doesn’t occur at the same time7. Even 10-15% of protein-encoding genes are regulated by our day-night rhythms8. There is a brain region that controls all these peripheral clocks called the suprachiasmatic nucleus (SCN). It is a cluster of only 20,000 neurons, located in the hypothalamus9 at the base of the brain opposite your eyes. When an animal lacks this timekeeper, it sleeps and wakes at random intervals during the day and likewise, degeneration of the SCN in patients with late-stage Alzheimer’s Disease lose their sense of time10.
You might wonder… if the SCN keeps our daily rhythms in check, what keeps the SCN under control? SCN neurons receive and are regulated by many inputs, however, the most important Zeitgeber (German for something that “gives time”), is light11. The presence or absence of light in the environment guides systems in our body to promote wakefulness or sleep. Light entrains the circadian clock to the light-dark cycle using melanopsin, a photopigment in the retinal cells of the eye12. The SCN receives information from the eyes and SCN neurons alter their firing patterns according to light so it can communicate this to the rest of the body.
The only reason for time is so that everything doesn’t happen at once.
Albert Einstein
The SCN structure has neurons that send their neuronal projections to the pineal gland which is responsible for secreting melatonin. Melatonin is the “sleep hormone” or over-the-counter supplement some people take to cure their jetlag13. Early in the morning, retinal cells sense bright light in the blue-spectrum and communicate this to the SCN which re-sets the clock. The SCN inhibits melatonin hormone production and in turn alters genes in cells of different organs in the body. You can digest your breakfast because cortisol is released from the adrenal glands; dictating metabolism and repair (Figure 1).
In order to survive, animals are primed to anticipate night and day to determine if and when they should run or rest. Viewing light at certain times of the day has evolved over thousands of years to align our attention, preparedness to move, learn and socialize, with daylight. However, in modern life, we are bombarded by artificial light, with unbroken access to our phones and the pressure to keep up with a 24/7 news cycle. In the evening, this extended period of light sends false signals to the brain and melatonin levels cannot build up. Besides, we are currently spending most of our time indoors under lock-down, and therefore unable to get enough bright light in the morning to reset our internal clocks. These conflicting signals have resulted in disrupted circadian rhythms.
Can we be ‘smart’ with our smartphones?
Even though it has been established that viewing bright screens at all hours of the day is negatively impacting our health, technology is something we cannot escape. Instead, we can utilize social media to deepen our understanding of the brain and how to optimize mental health. A study investigating the behavior of Twitter users looked at 800 million tweets over 4 years in the UK and highlighted distinctive circadian patterns for positive and negative moods14. Furthermore, a new app, myCircadianClock, is tracking the daily timing of behaviors (eating, sleeping, moving) to help researchers in Satchin Panda’s group disentangle how our daily rhythms influence health outcomes. So even though apps are partly responsible, they might also help us in the future.
Our strong relationship to the solar day makes sense when you consider the important role that sunlight has had in the survival of our species. What is puzzling, is that the emergence of artificial light has led us to think we can somehow trick the power of that connection. But what extensive research shows is that we need to maintain the right balance, by becoming aware of the dark side of light. On the individual level, there are several things you can do to keep in sync with the sun and the stars; such as viewing natural bright light in the early morning, avoiding screens at night, and maintaining a consistent sleep regime. So watch your TikToks, but don’t let them set your internal clock.
About the writer
Jenna Pfeifer is a first-year neuroscience masters student, currently investigating the pathophysiology of the alpha-synuclein protein in skin and brain tissue of Parkinson’s disease patients. Jenna is interested in many fields from stem cell biology to network neuroscience and hopes to incorporate her interests to better understand the interaction between lifestyle and brain functioning.
Further reading
[1] Foster, R., & Kreitzman, L. (2004). The biological clocks that control the daily lives of every living thing: rhythms of life. [2] M. Lunn et al., “Health Consequences of Electric Lighting Practices in the Modern World: A Report on the National Toxicology Program’s Workshop on Shift Work at Night, Artificial Light at Night, and Circadian Disruption,” Science of Total Environment 607–8 (2017): 1073–84. [3] Parent, J., Sanders, W., & Forehand, R. (2016). Youth screen time and behavioral health problems: The role of sleep duration and disturbances. Journal of developmental and behavioral pediatrics: JDBP, 37(4), 277. [4] Lam, R. W., Levitt, A. J., Levitan, R. D., Michalak, E. E., Cheung, A. H., Morehouse, R., … & Tam, E. M. (2016). Efficacy of bright light treatment, fluoxetine, and the combination in patients with nonseasonal major depressive disorder: a randomized clinical trial. JAMA psychiatry, 73(1), 56-63. [5] Smarr, B. L., & Schirmer, A. E. (2018). 3.4 million real-world learning management system logins reveal the majority of students experience social jet lag correlated with decreased performance. Scientific reports, 8(1), 1-9. [6] Ray, S., & Reddy, A. B. (2020). COVID-19 management in light of the circadian clock. Nature Reviews Molecular Cell Biology, 21(9), 494-495. [7] Mohawk, J. A., Green, C. B., & Takahashi, J. S. (2012). Central and peripheral circadian clocks in mammals. Annual review of neuroscience, 35, 445-462. [8]Masri, S., Rigor, P., Cervantes, M., Ceglia, N., Sebastian, C., Xiao, C., … & Baldi, P. (2014). Partitioning circadian transcription by SIRT6 leads to segregated control of cellular metabolism. Cell, 158(3), 659-672. [9] Welsh, D. K., Takahashi, J. S., & Kay, S. A. (2010). Suprachiasmatic nucleus: cell autonomy and network properties. Annual review of physiology, 72, 551-577. [10] Weldemichael, D. A., & Grossberg, G. T. (2010). Circadian rhythm disturbances in patients with Alzheimer’s disease: a review. International Journal of Alzheimer’s Disease, 2010. [11] Blume, C., Garbazza, C., & Spitschan, M. (2019). Effects of light on human circadian rhythms, sleep and mood. Somnologie, 1-10. [12] Panda, S., Sato, T. K., Castrucci, A. M., Rollag, M. D., DeGrip, W. J., Hogenesch, J. B., … & Kay, S. A. (2002). Melanopsin (Opn4) requirement for normal light-induced circadian phase shifting. Science, 298(5601), 2213-2216. [13] Cajochen, C., Kräuchi, K., & Wirz‐Justice, A. (2003). Role of melatonin in the regulation of human circadian rhythms and sleep. Journal of neuroendocrinology, 15(4), 432-437. [14] Dzogang, F., Lightman, S., & Cristianini, N. (2017). Circadian mood variations in Twitter content. Brain and neuroscience advances, 1, 2398212817744501.Image credits: the cover photo and Figure 1. were custom-made using bioRender.