the development of diabetes-(16), and increased hunger/appetite (which could promote obesity -(17) In addition, information obtained from questionnaires in large sample groups has also shown statistical associations between chronically reduced sleep duration and increased risk of hypertension (particularly in women) (18), (19); diabetes(20); and weight gain(21), (22), (23). These results are particularly meaningful because they agree with the results of studies that examined these issues by experimentally reducing the participants' time in bed. There is also a suggestion that sleep restriction may adversely affect immune function. In one study, young adults who had been given a flu shot after four nights of reduced sleep had less than half of the antibody response 10 days later compared with individuals having normal sleep at the time of vaccination (24). Changes in these clinical markers after only a few nights of partial sleep loss typically reverse when normal sleep is allowed. However, it is important to recognize that many of the above studies examined the effects of relatively short-term alteration in sleep duration but many individuals in the community may have chronic partial sleep deprivation. The consequences and reversibility of any consequences in this regard are not known.
The results of studies done to date imply that numerous health consequences could be associated with sleep restriction. In contrast, increased sleep duration has not been associated with immediate negative health consequences. However, controlled long-term studies remain to be done.
The relationship between sleep duration and lifespan has been most frequently been examined as part of large health screens given to thousands (or millions) of individuals. Respondents are typically asked, "How many hours do you usually sleep each night?" and the association between the responses and mortality at a later date is evaluated. Two such recent large studies and a summary of 23 such studies from the past 30 years have shown that, compared with individuals with a 7-8 hour sleep duration, there is an increased risk of dying in individuals who reported a short sleep duration (usually substantially less than 7 hours) and in individuals who have reported a long sleep duration (generally