Although chronic sleep restriction is more common in the normal population and thus offers a more accurate depiction of real life conditions, total SD has been more thoroughly explored.Д¬ognitive performances measured in SD studies have included several domains.

In chronic partial SD, subjects are allowed restricted sleep time during several consecutive nights. In acute total SD protocols, the subjects are kept awake continuously, generally for 24вЂ“72 hours. Sleep deprivation is a study design to assess the effects of sleep loss. People who are exposed to sleep loss usually experience a decline in cognitive performance and changes in mood (for meta-analyses, see Pilcher and Huffcutt 1996 Philibert 2005). Immune response may be impaired and metabolic changes such as insulin resistance may occur (for review, see Spiegel et al 2005). Sleep loss, instead, seems to activate the sympathetic nervous system, which can lead to a rise of blood pressure ( Ogawa et al 2003) and an increase in cortisol secretion ( Spiegel et al 1999 Lac and Chamoux 2003). In addition, sleep is essential for cognitive performance, especially memory consolidation ( Maquet 2001 Stickgold 2005). Sleep is considered to be important to body restitution, like energy conservation, thermoregulation, and tissue recovery ( Maquet 2001). There are many unanswered questions regarding both the functions of sleep and the effects of sleep loss. Although revised вЂњthree-process modelsвЂќ (eg, Akerstedt and Folkard 1995 Van Dongen et al 2003b Achermann 2004) have been suggested, this classical model is the principal one used for study designs in SD research.

The interaction of these two processes determines the sleep/wake cycle and can be used to describe fluctuations in alertness and vigilance. The theory for circadian process C suggests a control of an endogenous circadian pacemaker, which affects thresholds for the onset and offset of a sleep episode. The homeostatic process S depends on sleep and wakefulness the need for sleep increases as wakefulness continues. Sleep is regulated by two processes: a homeostatic process S and circadian process C (eg, Achermann 2004). The average sleep length is between 7 and 8.5 h per day ( Kripke et al 2002 Carskadon and Dement 2005 Kronholm et al 2006). The need for sleep varies considerably between individuals ( Shneerson 2000). We concentrate on the studies that have been published since 1990. The effects of aging and gender, as well as interindividual differences are discussed. The emphasis on acute total SD reflects the quantity of studies carried out compared with partial SD. In this review, we separately consider the effects of acute total and chronic partial SD and describe the effects on cognitive performance. Particularly, results on the recovery from SD have suggested different physiological processes. Although both conditions induce several negative effects including impairments in cognitive performance, the underlying mechanisms seem to be somewhat different. When considering the effects of sleep loss, the distinction between total and partial SD is important. Furthermore, people tend to stretch their capacity and compromise their nightly sleep, thus becoming chronically sleep deprived. In such fields, the effect of acute total sleep deprivation (SD) on performance is crucial. Some professions such as health care, security and transportation require working at night. In certain jobs, people face sleep restriction. Working hours are constantly increasing along with an emphasis on active leisure. One important yet underestimated cause for that is sleep loss ( National Sleep Foundation 2007). Cognitive recovery processes, although insufficiently studied, seem to be more demanding in partial sleep restriction than in total SD.ДЄ personвЂ™s quality of life can be disrupted due to many different reasons. In addition to coping with SD, recovering from it also deserves attention.

Also interindividual differences in responses are substantial. Coping with SD depends on several factors, especially aging and gender. Studies on its effects on more demanding cognitive functions are lacking. Partial SD is found to influence attention, especially vigilance. First and foremost, total SD impairs attention and working memory, but it also affects other functions, such as long-term memory and decision-making. Both total and partial SD induce adverse changes in cognitive performance. Although the latter is more common in everyday life, the effects of total SD have been examined more thoroughly. Prolonged wakefulness can be due to acute total sleep deprivation (SD) or to chronic partial sleep restriction. Nevertheless, in the field of sleep and wakefulness, several unanswered questions remain. Today, prolonged wakefulness is a widespread phenomenon.

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