Aljohara S Almeneessier,1,2 Ahmed S BaHammam1 1Department of Medicine, University Sleep Disorders Center, College of Medicine, King Saud University, Riyadh, Saudi Arabia; 2Department of Family Medicine, College of Medicine, King Saud University, Riyadh, Saudi Arabia Abstract: Mealtimes and feeding schedules may interfere with the circadian system and impact sleep. The practice of intermittent fasting (IF) in its different formats is increasing worldwide. However, most studies addressing the effect of IF on circadian rhythms, daytime sleepiness, and sleep architecture have been conducted during diurnal IF for Ramadan. In this article, we analyze the effect of diurnal IF on the circadian clock, sleep, and daytime sleepiness. In free-living, unconstrained environments that do not control for lifestyle changes such as sleep/wake schedules, sleep duration, and light exposure, studies have demonstrated sudden and significant delays in bedtime and wake time during diurnal intermittent fasting for Ramadan. However, subsequent studies that accounted for lifestyle factors and sleep/wake patterns have reported no changes in markers of the biological clock, daytime sleepiness, or sleep parameters. Nevertheless, several researchers have demonstrated a reduction in the proportion of rapid eye movement stage sleep as the significant alteration in sleep architecture during fasting. Keywords: mealtime, Ramadan, chronotype, sleep architecture, alertness, food, light, caloric restriction