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.
