<P> Scientists have been investigating the benefits of napping for years: the 30 - minute nap, as well as sleep durations of 1--2 hours . Performance across a wide range of cognitive processes has been tested . Studies demonstrate that naps are as good as a night of sleep for some types of memory tasks . A NASA study led by David F. Dinges, professor at the University of Pennsylvania School of Medicine, found that naps can improve certain memory functions and that long naps are more effective than short ones . In that NASA study, volunteers spent several days living on one of 18 different sleep schedules, all in a laboratory setting . To measure the effectiveness of the naps, tests probing memory, alertness, response time, and other cognitive skills were used . </P> <P> The National Institute of Mental Health funded a team of doctors, led by Alan Hobson, Robert Stickgold, and colleagues at Harvard University for a study which showed that a midday nap reverses information overload . Reporting in Nature Neuroscience, Sara Mednick, Stickgold and colleagues also demonstrated that, in some cases, a 1 - hour nap could even boost performance to an individual's top levels . The NIMH team wrote: "The bottom line is: we should stop feeling guilty about taking that' power nap' at work ." </P> <P> The siesta habit has recently been associated with a 37% reduction in coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep (Naska et al., 2007). Nevertheless, epidemiological studies on the relations between cardiovascular health and siesta have led to conflicting conclusions, possibly because of poor control of moderator variables, such as physical activity . It is possible that people who take a siesta have different physical activity habits, e.g. waking earlier and scheduling more activity during the morning . Such differences in physical activity may mediate different 24 - hour profiles in cardiovascular function . Even if such effects of physical activity can be discounted for explaining the relationship between siesta and cardiovascular health, it is still unknown whether it is the daytime nap itself, a supine posture or the expectancy of a nap that is the most important factor . It was recently suggested that a short nap can reduce stress and blood pressure (BP), with the main changes in BP occurring between the time of lights off and the onset of stage 1 (Zaregarizi, M. 2007 & 2012). </P> <P> Zaregarizi and his team have concluded that the acute time of falling asleep was where beneficial cardiovascular changes take place . This study has indicated that a large decline in blood pressure occurs during the daytime sleep - onset period only when sleep is expected; however, when subjects rest in a supine position, the same reduction in blood pressure is not observed . This blood pressure reduction may be associated with the lower coronary mortality rates seen in Mediterranean and Latin American populations where siestas are common . Zaregarizi assessed cardiovascular function (blood pressure, heart rate, and measurements of blood vessel dilation) while nine healthy volunteers, 34 years of age on average, spent an hour standing quietly; reclining at rest but not sleeping; or reclining to nap . All participants were restricted to 4 hours of sleep on the night prior to each of the sleep laboratory tests . During three daytime naps, he noted significant reductions in blood pressure and heart rate . By contrast, the team did not observe changes in cardiovascular function while the participants were standing or reclining at rest . </P>

How many hours of sleep is considered a nap