Sleep and Circadian Rhythm

Written By Teena Thomas

Chapters:

1. Introduction

2. Basics of Circadian Rhythm

3. How does the biological sex of an individual affect their sleep cycle?

4. Correlation between hormone changes and sleep cycle

5. Conclusion

6. Cited Sources

I. Introduction

Biologically, the concept of time is not just numerical, but rather a physical phenomenon that our bodies have adapted to. As humans, (and all living creatures for that matter), our bodies have adapted to a specific schedule, which is responsible for the timeline one rests and lays awake. That schedule is also known as the Circadian Rhythm —a biological clock that governs the ebb and flow of our physiological and behavioral processes. This report entails the basics of Circadian Rhythm, its substantial importance, and an outlook on the various factors that contribute to the way one’s sleep schedule functions.

II. Basics of Circadian Rhythm

According to the National Institute of General Medical Sciences, “[c]ircadian rhythms are physical, mental, and behavioral changes that follow a 24-hour cycle” (1, NIGMS). It is a natural process that controls the sleep cycle of an organism. Typically, an individual’s circadian rhythm is “programmed” to react based on external stimuli, such as the light of day. To elaborate, the human body naturally invokes a sense of drowsiness when the environment is dark, or more specifically when it is night outside, but when it is light outside/midday, our bodies naturally stay awake and alert. For example, “exercise, hormones, [and] medication” (UCLA Health) all constitute the functions of the circadian rhythm. Additionally, circadian rhythms are said to be regulated by biological clocks, which are “an organism’s natural timing devices,” (2, NIGMS) and are “composed of specific molecules that interact with cells throughout the body” (2, NIGMS). Furthermore, it is important to note that similarities have been identified between many different species, ranging from humans to fruit flies on the composition of the biological clock. Concurrently, the National Institute of General Medical Sciences states that within the network of cells that contribute to the circadian rhythm, there lies a master clock. In this case, the master clock isn’t a grandfather clock (haha), but rather the control center of every biological clock that resides within an organism. By keeping each clock in sync, the master clock acts as the facilitator for circadian rhythms, allowing the implementation of its symptoms to occur in a timely manner. For further understanding, one can refer back to the hypothalamus, where a structure (suprachiasmatic nucleus) resides and “receives direct input from the eyes” (3, NIGMS) to form the master clock.

III. How does the biological sex of an individual affect their sleep cycle?

Since an individual’s circadian rhythm is generally affected by both internal and external stimuli, a crucial point that has been argued is that the biological sex of an individual affects their sleep cycle. According to recent research studies, “the circadian rhythms of melatonin and body temperatures are set to an earlier hour in women than men, even when the women and the men…[have] consistent bedtimes and wake times” (1, Duffy). The reason for this? Unknown. But, various sets of data have pointed to similar conclusions. After a group of individuals was studied on their intrinsic circadian period, it was reported that the ICP was “significantly shorter in women…and that a significantly greater proportion of women have intrinsic circadian periods shorter than 24.0 h” (1, Duffy). These results are notable because decreased intrinsic circadian periods have shown a correlation to the prevalence of insomnia in adults 18+. On the other hand, men’s ICP appeared to be adjacent to 24 hours, contrary to the women’s results. In a way, these results are quite ironic as the average life expectancy is almost always higher for women, hinting at a possible connection between the two.

IV. Correlation between Hormone Changes and Sleep Cycle

The National Institute of Health states that “the regulation and metabolism of several hormones are influenced by interactions between the effects of sleep and the intrinsic circadian system; growth hormone, melatonin, cortisol, leptin, and ghrelin levels are highly correlated with sleep and circadian rhythmicity.”

V. Conclusion

In conclusion, it is clear that circadian rhythms are more than just timekeepers; they are also the facilitators of a workforce that permeates every cell and living thing on our planet. This report's pages have illuminated the significant ways in which these biological clocks affect our health, behavior, and well-being.

Cited Sources

“Circadian Rhythms.” National Institute of General Medical Sciences, U.S. Department of Health and Human Services, nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx#:~:text=%E2%80%8B%E2%80%8BWhat%20are%20circadian,the%20study%20of%20circadian%20rhythms. Accessed 30 Aug. 2023.

“Circadian Rhythms.” UCLA Health System, University of California, Los Angeles, www.uclahealth.org/medical-services/sleep-disorders/patient-resources/patient-education/circadian-rhythms. Accessed 30 Aug. 2023.

Duffy, Jeanne F. “Sex Difference in the Near-24-Hour Intrinsic Period of the Human ... - PNAS.” PNAS, PNAS Consultive Review Pilot, 17 Mar. 2011, www.pnas.org/doi/10.1073/pnas.1010666108.

“Circadian Rhythms.” National Institute of General Medical Sciences, U.S. Department of Health and Human Services, nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx#:~:text=%E2%80%8B%E2%80%8BWhat%20are%20circadian,the%20study%20of%20circadian%20rhythms. Accessed 30 Aug. 2023.