Sleep inertia is a physiological state characterised by a decline in motor dexterity and a subjective feeling of grogginess immediately following an abrupt awake

Awake

Wakefulness is a daily recurring brain state and state of consciousness in which an individual is conscious and engages in a coherent cognitive and behavior responses to the external world such as communication, ambulation, nutritional ingestion and procreation...

ning. 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

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

.

Factors

NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 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. 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
  Circadian rhythm
  A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...
  
  . Waking during a drop in body temperature tends to produce more sleep inertia.
• Chemical influences. Studies have shown that drugs such as caffeine
  Caffeine
  Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants...
  
   can suppress the effect of sleep inertia, possibly by blocking adenosine receptors in the brain
  Brain
  The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
  
  .

Sleep inertia can be more severe and last longer when a nap follows a prolonged period of wakefulness or an accumulated sleep debt

Sleep debt

Sleep debt or sleep deficit is the cumulative effect of not getting enough sleep. A large sleep debt may lead to mental and/or physical fatigue....

. 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.

If a subject is awakened due to a perceived danger the duration of sleep inertia may be reduced to only a few seconds.

Cause

One theory is that sleep inertia is caused by the build-up of adenosine in the brain during NREM sleep. Adenosine then binds to receptors, and feelings of tiredness result.

External links

• Flying Safely Article on Napping