older individuals). The amount of sleep then decreased to about 9 total hours in the younger group including about an hour in the nap and 7 hours in the older participants, also including about an hour in the nap. These data demonstrate a large difference in maximal ability to sleep as a function of age, and an inability to extend sleep beyond these values despite spending 16 hours per day in bed. The younger and older groups had reported habitual sleep times of 8 and 6 - 7 hours, respectively, upon entry into the study. Therefore, both groups slept about an hour longer in the isolation environment but this was at the expense of spending almost eight additional hours in bed. In the real world, it is likely that time spent attempting to sleep is determined by the balance between "cost and benefit," where the benefits in alertness and performance associated with increased sleep are balanced by the cost of spending additional time in bed (and, perhaps, additional time awake in bed).
Researchers have also identified people who had short (6 hours or less) or long (9 hours or more) sleep requirements throughout much of their lives (32). In general, long sleep patterns are more stable and persistent throughout life while short sleep patterns more often begin in the late teens (33). Both long and short sleepers became sleepier and have poor performance after total sleep deprivation (34), although some of the short sleepers may have been partially sleep deprived (35). Other research has identified rare extremely short sleepers who have documented sleep of less than three hours per night but do not display daytime sleepiness or performance decrements associated with chronic sleep loss (36), (37). The existence of extremely short sleepers has led to the hypothesis that sleep could be an instinct rather than a restorative process (38). There is little recent support for this theory, but sleep durations are widely divergent across animal species and our knowledge of sleep function remains limited.
There are significant mood, performance, health, and mortality consequences associated with sleep restriction, and these