Sleep inertia is a physiological state characterised by a decline in motor dexterity and a subjective feeling of grogginess immediately following an abrupt awakening. The impaired alertness may interfere with the ability to perform mental or physical tasks. Sleep inertia can also refer to the tendency of a person wanting to return to sleep.
Sleep inertia occurs normally after awakening. Upon awakening in the morning, subjective alertness and mental performance are significantly impaired. Morning sleep inertia takes two to four hours to dissipate completely, though subjects may function adequately before that time.
- Depth of sleep when awakened. After roughly 10–30 minutes, the brain enters into slow-wave sleep. Being awakened during this stage yields more sleep inertia than awakening from other stages of sleep. However, since one cannot predict accurately the stages of sleep that will occur within a nap, there is no physiological reason to try to limit the length of a nap. When one is sleep deprived, any sleep is good.
- Timing of sleep. Sleep inertia is thought to be related to the phase of the body's circadian rhythm. Sleep inertia in terms of a serial addition task had a strong circadian rhythm. "Sleep inertia is the impaired cognitive performance immediately upon awakening, which decays over tens of minutes. This phenomenon has relevance to people who need to make important decisions soon after awakening, such as on-call emergency workers."
- Chemical influences. Drugs such as caffeine can suppress the effect of sleep inertia, possibly by blocking adenosine receptors in the brain. Sustained low-dose caffeine was examined as a sleep inertia countermeasure during the last 66 hours of an 88-hour period of wakefulness that included seven two-hour naps. Lapses in attention were examined after awakening from naps. Performance was impaired significantly in the placebo condition but not with caffeine, and caffeine had only modest effects on nap sleep structure.
Reaction time performance is directly related to sleep stage at awakening; persons awakened during the deepest sleep have the slowest reaction times.
Testing of mental arithmetic capability after one- and two hour naps at all times of day and night and after varying amounts of sleep and sleep deprivation demonstrated an inertia characterized by social interaction but with simultaneous performance impairment, reverie and misjudgment of sleepiness.