It is something that you may experience nearly every morning when you wake up: that compelling, nearly irresistible desire to go back to sleep. Even after you get up, you may feel groggy and ready to return to bed. This is called sleep inertia, and it can make it very difficult to wake up and function at your best.
Sleep inertia was first described among U.S. Air Force pilots in the 1950s. Pilots were often stationed in the cockpits of their planes, ready to take off at a moment’s notice. It was found that if these pilots were asleep when the alarm sounded, they’d awaken and make simple mistakes, their minds still groggy from being asleep.
Inertia refers to the concept in physics that an object naturally resists changes in its state of motion. A ball rolling down a hill will continue to roll, and one at rest tries to remain at rest, unless other forces are applied to alter their state. As the concept of inertia is applied to sleep, when you are asleep, your brain would just as soon stay asleep.
This phenomenon leads to sleepiness and cognitive and psychomotor impairment that can occur immediately after awakening. Though most of us aren’t flying fighter jets, we may be impaired in our ability to make decisions or perform complex activities like driving a car, and we may have a feeling of profound mental grogginess.
These symptoms most commonly will occur with abrupt awakenings, especially from deep or slow-wave sleep or when sleep duration is insufficient. The symptoms may persist for minutes up to an hour or more after awakening. Though it is not fully understood, one theory suggests that sleep inertia is caused by the build up of a neurotransmitter called adenosine within the brain during non-REM sleep leading to feelings of tiredness. It may be worsened in sleep disorders such as sleep apnea and delayed sleep phase syndrome.