Altered sleep-wake cycles and food intake: the Ramadan model

In this review the effects of diurnal fasting on normal physiological processes are considered. Ramadan is placed in a circadian context, food and fluid ingestion being displaced to the pre-sunrise and post-sunset hours. Over the holy month, negative energy balance is often experienced, though this deficit is not a universal finding. Responses to exercise during the day show influences consistent with hypohydration and an increased reliance on fat as a source of fuel for exercise. Muscle performance and psychomotor performance are impaired as the month of fasting progresses but it is not clear how circadian rhythms in responses to activity are altered. For some measures at rest there is a reduction in amplitude and a delay in acrophase. Health-related benefits are reflected in a rise in high-density lipoprotein cholesterol and individuals with predispositions for coronary heart disease are not at increased risk of cardiovascular disorders due to fasting. The physiological adjustments during the month have some similarities to the disturbances in circadian rhythms experienced in different circumstances. The Ramadan model provides an alternative to those for ageing, nocturnal shift-work and time-zone transitions in understanding the links between behaviour and endogenous circadian rhythms.