Naska et al1 report that siesta or daytime napping is associated with reduced coronary mortality and has stronger protective effects among those who work. They demonstrate that this effect is independent of potential confounders such as differences in physical activity or diet. Unfortunately, Naska et al1 fail to consider overall sleep habits in their work. Short-term sleep deprivation has been shown to have important effects on inflammatory, metabolic, and neuronal pathways including elevations in C-reactive peptide level, insulin resistance, and sympathetic activity, which may promote development of coronary disease.2,3 In addition, long-term studies suggest that sleep duration is an independent predictor for both incident cardiovascular disease and mortality, exhibiting a U-shaped association, with the minimum risk in those obtaining 7 to 8 hours of sleep.4,5 Thus, the protective effects of daytime naps may be mediated by increasing total sleep time among those who do not obtain sufficient sleep at night. Because job requirements frequently force individuals to wake up earlier than they would like in the morning, the finding of a stronger beneficial effect of the siesta among employed individuals may simply reflect insufficient nocturnal sleep in this group. Conversely, in individuals who obtain 8 or more hours of sleep at night, additional sleep during the day may actually increase coronary risk. Thus, population differences in nocturnal sleep habits may also explain the conflicting findings between this study and prior work on siesta. Clearly, to fully understand the health effects of daytime naps, a comprehensive assessment of sleep habits is required.
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