On March 15, 2022, the US Senate passed a bill making daylight savings time (DST) the nation’s permanent time, starting in November 2023. With this law, the country would no longer have to switch between daylight savings time and standard time twice a year. At first glance, this idea seems like a win-win situation. What could be the downside to a law that can save millions of families the hassle of toggling their clocks forwards and backwards? How could anyone be opposed to potentially reducing the incidence of time-change related car accidents [1]? Even better, what could go wrong when some of the groggiest mornings of the year are taken away?

However, this is not the first time the Sunshine Protection Act has been proposed. The US has previously tried and failed to maintain daylight savings time year round. People have claimed that it would reduce energy consumption, maximize daylight, and provide more time for both productivity and enjoyment of the day. Yet, data does not support the fact that following DST has led to significant energy conservation, and following DST year round would not maximize daylight in any season other than Summer [2]. Furthermore, experts in sleep medicine fiercely disagree with the decision to keep daylight savings time over standard time [3]. While a number of practical, legal, economic, and safety considerations fuel this debate, the crux of the issue, and the science behind the right answer, hinge on the circadian rhythm. So, what is it?

The Circadian Rhythm: What is it?

The circadian rhythm is only one of the many biological cycles that guide our daily lives. Defined by their approximate durations, the most common rhythms are circadian, roughly 24 hours; ultradian, under 24 hours; and infradian, more than 24 hours [4,5]. We see a colorful variety of examples of each in ourselves and in the world around us. For some animals, reproductive cycles, hibernation, and migration are circannual rhythms, lasting about a year [6]. In human females, the menstrual cycle is infradian, lasting approximately a month. On the other hand, many hormonal fluctuations are ultradian, and, most relevantly, our patterns of activity and rest follow a circadian rhythm [Figure 1,6]. Other key qualities of these biological rhythms are their endogenous, or internally originating, nature and ability to be observed through body processes. 

To provide a brief history of how scientists learned about circadian rhythms, we can look to the year 1938, when Nathaniel Kleitman conducted studies on human cave dwellers [7]. In the absence of external cues like light or temperature, which indicate the time of day, Kleitman found that the cave dwellers’ body temperatures still fluctuated according to an almost 24-hour period. What’s more, when the cave dwellers were told to follow a 28-hour rest-activity schedule, their bodies’ circadian rhythms still lasted just over 24 hours. Later observations of people living underground in German bunkers for multiple months on end revealed that important functions such as body temperature, urine volume, and sleep/wake rhythms all seemed to fluctuate according to these 24-hour cycles [7]. Moreover, comparisons of blind and sighted individuals helped further refine the estimation of the length of human circadian periods to roughly 24.3-hours [7,8]. 

Circadian Science: Order and Disorders

While the absence of fluctuating light and temperature cues does not change the length of people’s circadian periods, it does affect their synchronization to the time of day [Figure 1]. Light cues, specifically, are capable of resetting the body’s circadian rhythm. Under normal exposure to light and darkness, research participants were active in the day and restful at night [Figure 1]. When people experience constant change in light and temperatures, however, researchers observed that the circadian rhythm began to start later and later in the day [Figure 2]. As a result, even when the circadian period remains constant at about 24.3 hours, these isolated rest and activity cycles gradually slide to different times of day. In some blind people, especially those who become completely incapable of perceiving light, their circadian rhythm varies in length compared to the sighted. Their sleep/wake cycles may also slide, just like those of people who have been wholly isolated from normal cues [8,9]. 

One circadian rhythm disorder where this out-of-sync, non-24 hour cycle is observed falls under the same name: “Non-24 Sleep–Wake Disorder.” Non-24 and other sleep/wake disorders occur in up to 76% of blind people [9]. The disorder also occurs in sighted individuals, but those cases are usually triggered by working unusual shifts, jet lag, or traumatic brain injury. To treat non-24 and other instances of circadian rhythm disorders, patients are generally advised to find alternative cues for their circadian rhythms, natural phenomena known as zeitgebers [9]. For example, amongst the sighted and those with certain types of blindness, reintroduction of daylight and bright light exposure in the morning can reset the circadian rhythm back to normal [9]. Other options include routine stimuli, like alarm usage, or following a strict daily schedule.

Circadian Gears: Molecular Mechanisms

Through animal studies, scientists have been able to research how select brain regions, hormones, and genes play a role in regulating our circadian rhythm. Studies in flies, mice, cats, bacteria, and even sponges have revealed the core components of our circadian clocks: a) sensors to register environmental entrainment–referring to the synchronization of two rhythms (like the circadian rhythm with the light/dark cycle, or how two pendulum clocks hanging on the same wall will move in harmony), b) a brain region known as the the suprachiasmatic nuclei (SCN), and c) clock genes [10,11,12]. 

At the molecular level, scientists understand circadian rhythms through clock genes. Clock genes are transcribed into clock proteins, and together, they regulate the body’s biological cycles [11, Figure 1]. Some clock genes and proteins are light sensitive, meaning they behave differently in the presence or absence of light (e.g., start to be produced or decayed) [13]. Clock genes are expressed in all of the body’s cells. Different tissues and organs may have their own “local” clocks, which get synchronized by a central mechanism: the SCN in the brain [Figure 1]. 

The SCN detects light cues through a special photoreceptor in the eyes called melanopsin, allowing the SCN to regulate sleep, metabolism, and hormones [11,12]. The SCN is commonly referred to as a “master pacemaker,” which helps in synchronizinges local clocks across the body to its global rhythm [Figure 1, 14]. A useful analogy for these phenomena brings us back to the topic of the US time debate. Our master pacemaker may be thought of as the nation’s government, and it has to resolve the different time zones and demands across the country into one sensible system. Local clocks could theoretically operate independently, but it makes sense for every part of our country–from our respective occupations to utility services, or even the local bakery–to operate by the same time system. Imagine trying to buy a loaf of bread in the morning, only to find out that the bakery opens on DST, while you were following standard time! If we all followed different time systems, we would not be able to work or live together smoothly, on the same clock. The same can be said for our cells, organs, and body.

Timely Takeaways

Grasping how light, temperatures, clock genes, and the SCN regulate circadian rhythms helps us to recognize the importance of following time schedules. Living by a clock lets us respect our bodies’ biological cycles and maintain health. Although much of the conversation around the daylight saving time controversy is motivated by social and economic considerations, it is equally–if not more–important to recognize the scientific ones. Sleep medicine experts say that standard time more closely resembles our endogenous circadian rhythm, and that if we are to keep only one of either daylight saving time or standard time, standard time would be the more sensible, healthy choice. If we tried to follow DST all year, then there would be days in the winter when the sun would not rise until past 9:00 AM. Trying to wake up and function without light goes against our biological coding and may literally prevent our bodies from working as they should: from clock genes to the SCN, light plays a crucial role in our capacity to be awake and feel healthy. As in the case of jet lag, or shift workers, putting undue pressure on our bodies chronically may result in accumulated sleep debt, circadian disorders, and more serious health crises [3]. In order to pursue and enjoy anything at all, we must first listen to our body and prioritize our wellbeing. 

References

[1] Sood, N., & Ghosh, A. (2007). The short and long run effects of daylight saving time on fatal automobile crashes. BEJEAP, vol. 7(1). https://www.degruyter.com/document/doi/10.2202/1935-1682.1618/html 

[2] Malow, B. A., Veatch, O. J., & Bagai, K. (2020). Are daylight saving time changes bad for the brain?. JAMA Neurol., vol. 77(1), 9–10. https://doi.org/10.1001/jamaneurol.2019.3780 

[3] Rishi, M. A., Ahmed, O., Barrantes Perez, J. H., Berneking, M., Dombrowsky, J., Flynn-Evans, E. E., … & Gurubhagavatula, I. (2020). Daylight saving time: an American Academy of Sleep Medicine position statement. JCSM, vol. 16(10):1781–1784. https://doi.org/10.5664/jcsm.8780 

[4] Aschoff, J. (1981). A survey on biological rhythms. J. Biol. Rhythms, 3–10. Springer, Boston, MA. https://link.springer.com/chapter/10.1007/978-1-4615-6552-9_1 

[5] Laje, R., Agostino, P. V., & Golombek, D. A. (2018). The times of our lives: interaction among different biological periodicities. Front. Integr. Neurosci., vol. 12(10). https://doi.org/10.3389/fnint.2018.00010 

[6] Bailey A.M., Demas G.E. and Kriegsfeld L.J. (2014) Biological Rhythms. Reference Module in Biomedical Sciences. Elsevier. http://dx.doi.org/10.1016/B978-0-12-801238-3.03794-6 

[7] Czeisler, C. A., & Gooley, J. J. (2007). Sleep and circadian rhythms in humans. In CSHL: Symposia on Quantitative Biology, vol. 72:579–597. Cold Spring Harbor Laboratory Press. http://symposium.cshlp.org/content/72/579.short 

[8] LENScience.( 2022). What are Circadian Rhythms? – The University of Auckland. [online] <https://www.lenscience.auckland.ac.nz/en/about/teaching-and-learning-resources/senior-biology-learning-resources/circadian-rhythms-keeping-time/what-are-circadian-rhythms.html> [Accessed 24 June 2022].

[9] Quera Salva, M. A., Hartley, S., Léger, D., & Dauvilliers, Y. A. (2017). Non-24-hour sleep–wake rhythm disorder in the totally blind: diagnosis and management. Front. Neurol., vol. 8:686. https://doi.org/10.3389/fneur.2017.00686

[10] Jindrich, K., Roper, K. E., Lemon, S., Degnan, B. M., Reitzel, A. M., & Degnan, S. M. (2017). Origin of the animal circadian clock: diurnal and light-entrained gene expression in the sponge Amphimedon queenslandica. Front. Mar. Sci., vol. 4: 327. https://doi.org/10.3389/fmars.2017.00327

[11] Hardin, P. E. (2011). Molecular genetic analysis of circadian timekeeping in Drosophila. Adv. Genet., vol. 74:141–173. https://doi.org/10.1016/B978-0-12-387690-4.00005-2

[12] Takahashi, J. S. (2017). Transcriptional architecture of the mammalian circadian clock. Nat. Rev. Genet., vol. 18(3):164–179. https://doi.org/10.1038/nrg.2016.150

[13] Hastings, M. (1998). The brain, circadian rhythms, and clock genes. BMJ, vol. 317(7174):1704–1707. https://doi.org/10.1136/bmj.317.7174.1704 

[14] Lehr, A. B., McDonald, R. J., Thorpe, C. M., Tetzlaff, C., & Deibel, S. H. (2021). A local circadian clock for memory?. Neurosci. Biobehav. Rev., vol. 127:946–957. https://doi.org/10.1016/j.neubiorev.2020.11.032