Background: High process temperatures associated with industrial operations augment risk of heat stress and illness, particularly during summer months in the Gulf Region. Lack of hydration and nutrition during day time, during Ramadan can subject workers to even greater risk of heat stress and illness. Aims: To examine the physiological effects of prolonged fasting in thermally challenging conditions. Methods: Longitudinal measurements were carried out on employees during fasting in Ramadan in three departments of an aluminium smelter. After informed consent, physiological parameters were measured at 4-hour intervals. Results: Average heart rate and urine specific gravity increased in the first 4 hours of shift work, while tympanic temperature did not rise significantly. Moreover, in the second 4 hours of shift work, urine specific gravity stabilized compared to the first 4 hours. Conclusions: Robust workplace measures are needed for industries with high process temperatures, located in the Gulf Region, in order to minimize the enhanced risk of heat stress and illness during Ramadan.