Definition

Transient disorientation, grogginess and performance impairment that can occur after wakening. The length and intensity of sleep inertia is greatest when the individual has not had enough sleep, is woken from slow-wave sleep (non-REM stages 3 and 4) or woken during the Window of Circadian Low (WOCL).

Source: Fatigue Management Guide for ANSPs

Description

People tend to feel groggy for a period of time following awakening from a long sleep or nap. This condition of temporary disorientation and reduced performance is called "sleep inertia". It usually lasts from 30 to 60 minutes but can sometimes reach two hours. The duration tends to be longer after sleep deprivation. While the biological reason for the phenomenon has not been determined, some researchers hypothesize that this is a protective mechanism that helps maintain sleep after an unwanted waking.