The sleep inertia phenomenon during the sleep-wake transition: Theoreticaland operational issues

Sleep inertia (SI) defines a period of transitory hypovigilance, confusion, disorientation of behavior and impaired cognitive and sensory-motor perfor mance that immediately follows awakening. Si, the cognitive and behavioral correlate of the transition from sleep to wakefulness, has been incorporate d in several models of sleep and vigilance regulation. Monitoring of severa l physiological parameters during the awakening period clearly indicate tha t this transition process is very slow. On the cognitive and behavioral sid e, SI has relevant operational implications. SI is one of the most serious contraindications to the use of napping during quasi-continuous operations if the individual may be required to perform complex tasks immediately afte r sudden awakening at unpredictable times. The studies on SI modulating fac tors showed that SI is strongly affected by slow wave sleep amount and slee p depth, while the outcomes concerning the modulation of SI by circadian fa ctors are not consistent. Cognitive tasks involving high attentional load s eem to be much more affected by SI than simple motor ones, performance accu racy being more impaired than speed. Finally, some possible countermeasures against the detrimental effects of SI to be applied in operational setting s have been provided.