Some epidemiological studies have found an association between frequent daytime napping and increased risk of morbidity, mortality, and dementia. There is a relatively large literature describing the effects of prophylactic naps on behavioral, physiological, and psychological measures. Studies in which daytime sleep periods are prescribed, and analyses are directed at the effects of such naps, are likely to provide the most solid evidence about the therapeutic value of naps. On average, when compared to younger subjects, older individuals obtain almost 2 hours less sleep per night. Laboratory studies also strongly support the notion that humans have a biological tendency to nap. Even well-rested individuals report a significant increase in subjective sleepiness and a corresponding decrease in alertness during the afternoon, accompanied by shorter sleep onset latencies during this time. Night-time sleep measures were unaffected by the nap, including total sleep time, sleep efficiency, and percentage of slow wave sleep.