During the day, Paris is a bustling metropolitan city, housing thousands of people as they take to the streets on their way to work. Once the sun sets, however, I begin to understand why this is the City of Lights. The Eiffel Tower lights up much of the night sky, twinkling at the onset of each new hour. While daytime bakeries close, late-night restaurants and cafes open their doors to the evening-inclined general public. On late night walks back to my apartment, I never fail to notice Parisian couples lounging at an outdoor café, enjoying the nighttime air with a drink and a pastry. Here, it seems to be a widely recognized and embraced concept that Parisians are night-folk.
Long after the sun sets, the lights of Paris are still up and brighter than ever. View from a street corner blocks from the Eiffel Tower.
This is a far cry from the nighttime environment in my hometown, a city on the outskirts of the Metro Atlanta area. I was always used to complete darkness at night, with no late-night city life to brighten the night sky. Even in Atlanta lights don’t shine quite as brightly. While I think the Parisian street scene is charming and I absolutely adore its thriving nightlife, it has left me struggling to get to sleep in the city where the lights never fade. I’ve only been here a month, but I wonder how all of this consistent light affects the sleeping patterns of the average Parisian relative to the people living in smaller, less heavily lit areas. Does the constant exposure to bright city lights at night-time in cities like Paris result in later sleep cycles that significantly differ from populations living in areas with less nighttime light pollution?
To answer this question, it is important to understand how variation in sleep cycles is defined. People can be sorted into two groups based on their times of wakefulness and alertness – “morning” or “evening” type people. These distinctions refer to a person’s chronotype, which measures individual differences in lifestyle and alertness in the morning versus the evening (Sun 2019). Morning-type people are more likely to get up earlier and exercise more, while evening-types typically go to sleep later, eat later, and wake up later. Based on the social environment alone, Paris seems to encourage more evening-oriented people.
Both chronotypes depends on chemical signaling in the brain to fall asleep at their respective times. Sleep relies on a neurotransmitter known as melatonin, a hormone operating in synchronicity with the onset of nighttime darkness. Melatonin is important for regular and consistent sleep, and delays in its release are expected to play a role in sleep disorders such as insomnia (Shechter 2018). This means that any factor that influences the release of melatonin prolongs wakefulness at night and causes difficulties in getting to sleep. One factor that has continuously shown associations with these delays is presence of light during and leading up to sleep (Shechter 2018). If light is present on a consistent basis, the regular delay of melatonin can cause an adjustment in chronotype towards being more evening inclined. In addition to delaying melatonin production, light alerts parts of the brain that control initiation and maintenance of sleep. (Sun 2019).
As shown above, melatonin is active in the dark. Presence of light inhibits the melatonin pathway and delays sleep.
A study compared circadian patterns between residents of a rural town and members of a small urban area (Carvalho 2013). They had a similar question, and collection of data involved comparison of light exposure and sleep patterns, as well as a structured chronotype questionnaire. The results they obtained show a significant difference between rural and urban populations in spread of chronotypes: the rural subjects showed a predominantly morning chronotype while the urban subjects were predominantly evening-oriented.
As shown above, there is a significantly difference between rural and urban subjects with regards to chronotype.
Their reasoning was that rural workers are exposed to sunlight during the day while they work, which reinforces the need to sleep when the sun goes down. Since urban citizens typically do not work outside, in-the-sun jobs, they are not exposed to as much natural sunlight and do not have their sleep cycles naturally reinforced. Instead, they tend to work technology-based jobs where they are at a computer screen (Carvalho 2014). This results in increased intake of bright and unnatural light, which is independent of the actual daylight (as one can use a computer at night). This would align with the Parisian lifestyle of bright light that is inconsistent with the light of the sun, which tampers with the natural sleep cycle. This reasoning was a strength of the study and proved their conclusion with valid evidence.
A weakness of this study was that it failed to address the possibility of other factors that may have influenced their data. The results they received were very strong, but the influence of light could have been a correlation instead of a causation. They made no effort to test for other variables that may have impacted results, such as timing of social events, parents with young children who require continuous attention, and other non-light related factors.
Based on these results, it looks like the late-night hustle and bustle does have a neurological effect on sleeping patterns. While it doesn’t hurt to be on the computer past sunset, it is clear that continued light during nighttime delays melatonin release on a consistent scale, which can result in the shifting of sleep cycles to an evening chronotype. Turns out there may be some nighttime benefits to living in small towns after all!
References:
Carvalho, F. G., Hidalgo, M. P., & Levandovski, R. (2014). Differences in circadian patterns between rural and urban populations: An epidemiological study in countryside. Chronobiology International, 31(3), 442-449.
Shechter, A., Kim, E. W., St-Onge, M., & Westwood, A. J. (2018). Blocking nocturnal blue light for insomnia: A randomized controlled trial. Journal of Psychiatric Research, 96, 196-202.
Sun, J., Chen, M., Cai, W., Wang, Z., Wu, S., Sun, X., & Liu, H. (2019). Chronotype: Implications for sleep quality in medical students. Chronobiology International, 1-9.
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