Sleep deprivation can have significant effects on performance if sleep loss is significant and occurs for several days without recovery sleep. For example, many studies show that if eight-hour sleepers are restricted to four hours of sleep for as little as a few days, their performance deteriorates on tasks like problem solving, reaction time and memory. However, the magnitude of these effects depends upon the person, how much sleep is lost, motivation and the circumstances under which sleep loss takes place.
Some individuals show a remarkable tolerance for sleep loss, particularly if the person is motivated to cope with sleep loss (examples would be dealing with a crisis, caring for a newborn, etc.) or if the sleep loss occurs under positive circumstances (excitement, a vacation, a social event, etc.). Also, the effects of sleep loss are more pronounced when people lose half of their normal sleep but less pronounced when they only lose one or two hours. Therefore, an eight-hour sleeper will show little or only moderate impairment in daytime functioning if restricted to six hours of sleep but will show more significant impairment when only allowed to sleep only four hours. And for every study that reveals impaired performance after sleep loss, other studies find minimal or no effects. These findings suggest that the effects of sleep loss are not consistent or robust.
Researchers are beginning to realize that the results of sleep deprivation studies are also confounded by the effects of stress. Many such studies involve highly stressful conditions that are far from routine, therefore it may the stress or the experiment that produces impairment after sleep loss. Similarly, real-life sleep loss typically occurs as a result of stress so we don't know if the performance decrements after sleep loss are due to sleep loss itself or the stress that causes the sleep loss.
