Circadian rhythms are physical and behavioral changes that follow a 24-hour cycle and are generated by an endogenous circadian biological clock. They prepare organisms for expected changes in the environment by regulating the timing of biological functions such as gene expression, sleep, metabolism, and activity. Chronotypes define individual differences in sleep-wake cycles that reflect the phase of circadian clock timing relative to the photoperiod. Morning types start and end their day early compared to evening types, who go to sleep and wake up later. Misalignment of an individual's chronotype with the natural photoperiod, resulting from social schedules (e.g. work, school), jet lag, or illness is a health risk for cardiovascular, metabolic, oncological, and other disorders. Many studies have described chronotypes in Western and Asian populations, but there are few studies surveying chronotypes in Arab countries, and none from Yemen. Here we present the first data describing chronotypes from Yemen. We assessed chronotypes in a population of young adults using the Morning Evening Questionnaire (MEQ). Our results show a majority of subjects classified as neither morning nor evening type, modestly increasing morning-type tendencies with age, and no significant gender effect. A survey of chronotype studies from Arabic countries was consistent with our results from Yemen. Further studies of populations underrepresented in global surveys of chronotype are important for better understanding the role of geographic, social, and cultural determinants of individual variation in chronotype and circadian entrainment.
Ahmed et al. (Wed,) studied this question.