Differences in circadian patterns between rural and urban populations: An epidemiological study in countryside

Abstract

The physiological pattern of the sleep-wake cycle is influenced by external synchronizing agents such as light and social patterns, creating variations in each individual's preferred active and sleep periods. Because of the demands of a 24-h working society, it may be imperative for many people to adapt their sleep patterns (physiologically) to their daily activities. Therefore, we analyzed the difference in sleep patterns and chronobiological parameters between an essentially rural farming and urban small-town populations. We studied 5942 subjects (women, 67.1%, N = 3985; mean age, 44.3 ± 13.1 years), from which the chronotype, circadian sleep pattern, and period of light exposure were collected using the Munich Chronotype Questionnaire (MCTQ). A structured questionnaire was also made for collection of social and demographic information. Compared with the urban population (N = 3427, 57.7%), the rural population (N = 2515, 42.3%) presented a more predominantly early sleep pattern, as determined by the mid-sleep phase (rural: 2.26 ± 1.16; urban: 3.15 ± 1.55; t-test, p < 0.001). We also found less social jetlag (rural: 0.32; urban: 0.55; Mann-Whitney U test, p < 0.001) and higher light-exposure (rural: 9.55 ± 2.31; urban: 8.46 ± 2.85; t test, p < 0.001) in the rural population. Additionally, the rural population presented a higher prevalence of psychiatric disorders (rural: 156, 6.20%; urban: 165, 4.80%; Chi-square, p < 0.05), and a lower prevalence of metabolic diseases (rural: 143, 5.70%; urban: 225, 6.60%; Chi-square, p < 0.05). The significant difference in sleep parameters, chronotype, and light exposure between groups remained after multivariate regression analysis (r(2 )= 0.41, F = 297.19, p < 0.001, β = 1.208). In this study, there was a significant difference between the rural and urban populations in natural light exposure and sleeping patterns. Because of agricultural work schedules, rural populations spend considerable time outside that is an obligation related to work schedules. Our results emphasize the idea that latitude may not be the main factor influencing individual circadian habits. Rather, circadian physiology adapts to differences in exposure to light (natural and artificial) as well as social and work schedules.

Chris Cornell, the Black Hole Sun, and the Seasonality of Suicide

“…This rural increase has been found in France [6], Hungary [48], New Zealand [49], Sweden [49], and Austria [50], but not in Mexico [34]. The possibility that differences in sunlight and artificial light exposure might account for these urban-rural differences has not previously been considered [51, 52]. A number of studies have consistently found that, compared to individuals without an established psychiatric diagnosis, the spring seasonality of suicide is more pronounced in individuals with an established psychiatric diagnosis [53, 54], especially in individuals with a history of mood disorders [36, 55].…”

Distribution and heritability of diurnal preference (chronotype) in a rural Brazilian family-based cohort, the Baependi study

“…The observable difference is likely slightly exaggerated by the fact that the other two populations were younger than the Baependi one (average age 36.4 in London and 31.8 in São Paulo); however, this age difference is entirely insufficient to account for the major part of the variance (the London study indicates that the relationship between age and MEQ score corresponds to an average increase of 1 unit approximately every 3.8 years14). The difference in chronotype between rural and metropolitan populations is well known and have been described in human population samples living both in Brazil212223 and elsewhere2425, and even in songbirds26. The most obvious and likely strongest determinant of this difference is differential exposure to natural light during the day (particularly during the advancing window of the phase response curve) and artificial light (from indoor or outdoor sources) during the night (particularly during the delaying window of the phase response curve).…”

Young Adults’ Sleep Duration on Work Days: Differences between East and West

“…In industrialized societies where electrical lighting is abundant, exposure to sunlight during daytime is reduced, light exposure after sunset is increased, circadian timing is delayed, and endogenous circadian clock is less synchronized with the natural light–dark cycle relative to an environment with only natural light exposure (10). In addition, compared with individuals residing in rural areas, urban dwellers have less exposure to natural light and later sleep schedules (45). Apart from its effects on sleep timing, electrical lighting may also be a cause of insufficient sleep in industrialized societies (46).…”