Sleep inertia: Information from Answers.com

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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 to want to return to sleeping. Typically, sleep inertia lasts up to 3 hours for a night wake up and up to 90 minutes for a day wake up. If the subject is awakened due to a perceived danger, however, the duration of sleep inertia is reduced to only a few seconds.

1 Factors
2 Cause
3 See also
4 References
5 External links

Factors

NASA studies have shown that a variety of factors influence the severity and duration of sleep inertia. These include:

Depth of sleep when awoken. After roughly 30 minutes, the body enters into deep sleep, the deepest stage of non-rapid eye movement sleep (NREM). Being woken during this stage yields more sleep inertia than awakening from other stages of sleep.
Timing of sleep. Sleep inertia is thought to be related to the phase of the body's circadian rhythm. Waking during a drop in body temperature tends to produce more sleep inertia.
Chemical influences. Studies have shown that drugs such as caffeine can suppress the effect of sleep inertia, possibly by blocking adenosine receptors in the brain.

Sleep inertia can be more severe and last longer when a nap follows a prolonged period of wakefulness or an accumulated sleep debt. Sleep inertia can often be reversed by activity and noise as well as caffeine. Reaction time performance is directly related to sleep stage at awakening; persons awakened during the deepest sleep have the slowest reaction times.^[1]

Cause

One theory is that sleep inertia is caused by the build-up of adenosine in the brain during non-REM sleep. Adenosine then binds to receptors, and feelings of tiredness result.^{[citation needed]}