Sleep inertia refers to the feeling of grogginess most people experience after awakening. Sleep inertia can last from 1 minute to 4 hours, but typically lasts 15-30 minutes. During this period, you are at a reduced level of capacity and may have trouble doing even simple everyday actions. The level of impairment can be serious. Tests of simple arithmetic ability have found people are often 5 standards deviation below the average during the sleep inertia period.
Usually people fully "wake up" in a half hour at most. For many, the morning routine of a shower or making coffee helps them through the inertial sleep inertia period.
Most of the time for most people, sleep inertia is a normal part of life and is nothing to worry about. There are a couple times when it is important, however:
1) If someone has been very sleep deprived or has been woken from a deep sleep stage, the sleep inertia can be more severe than normal and may take longer to overcome. People should try to be aware of their sleep inertia and compensate for it. It has been shown that a person’s self "performance monitoring" - which usually catches errors - is off during periods of sleep inertia. Prophylactic naps are recommended for people in critical jobs where they must be awake during normal sleep time. When people wake up from naps they often experience sleep inertia.
2) Drowsy driving – a lot of drowsy driving accidents happen early in the morning – not when the driver has been up too long, but when he or she has recently awoken and is still suffering from sleep inertia. Sleep inertia in a driver behind the wheel can be very dangerous as the impairment of motor and cognitive functions and can affect a person's ability to drive safely.
Many factors are involved in the characteristics of sleep inertia. The duration of prior sleep can influence the severity of subsequent sleep inertia. Although most studies have focused on sleep inertia after short naps, its effects can be shown after a normal 8-hour sleep period. A critical factor is the sleep stage prior to awakening.
Awakening during deep sleep produces more sleep inertia than awakening in stage 1 or 2. Waking up during REM sleep is in between. Many people experience sleep inertia even after short naps. When a person has been sleep deprived, sleep inertia can last longer, even after a full night’s sleep. When there has not been sleep deprivation, the sleep inertia rarely lasts more than 30 minutes.
The inertia is stronger when the awakening occurs near the trough of the core body temperature as compared to its peak, but this is consistent with inertia being worse when the person wakes from slow-wave deep sleep. It has been shown that people awakened from a period of recovery sleep after sleep deprivation have worse sleep inertia than normal. This is also consistent with the finding that recovery sleep is rich in slow-wave sleep.
Advanced imaging studies have shown that cerebral blood flow returns to waking levels in the brainstem and thalamus first. After a while (15 minutes of so), the brain’s anterior cortical regions get back to normal blood flow for daytime. This more or less corresponds to the sleep inertia period.
From a behavioral standpoint, sleep inertia is easy to measure. Psychologists have tests like the Descending Subtraction Task; the Auditory Reaction Time task; and the Finger Tapping Task test that can show how mental agility has declined. It appears that some sections of the brain wake up and emerge from sleep faster than others, which accounts for the diminished vigilance and disorientation during this period. In tests EEG readings showed higher beta frequencies in the left anterior of the scalp when the awakenings happened from Stage 2 than from Stage R. The posterior show stronger delta wave activity and the anterior shows lower delta wave activity during the sleep inertia phase.
Tests have shown it is possible to mitigate inertia by increasing the light level in the bedroom for a half hour before wake time.
Caffeine has been shown to reduce the effects of inertia (hence that morning cup of coffee so many people need), which suggests that the neurotransmitter adenosine causes at least some of the inertia. See: (http://www.ncbi.nlm.nih.gov/pubmed/11683484
"Grogginess" doesn’t have a precise medical or scientific meaning, but we all know what it is: not being fully awake or at normal performance after a sleep. Sleep inertia is the preferred term among professionals. Everyone has groggy mornings at least once in a while, but we all differ in how we wake up and start the day. "Morning people" rarely experience grogginess, or if they do not for very long. Others have grogginess every day. Some sleeping pills cause morning grogginess, too, although this was more common with old-style sleeping pills like Halcion than the newer pills.
Sleep inertia is sometimes called "worn-out syndrome".
A study published in the Journal of the American Medical Association in 2006 found that severe morning grogginess can be more dangerous and debilitating than staying up all night. They're both dangerous, of course, but it is important that people with bad grogginess do something to remedy it before they get behind the wheel. The mental handicap caused by sleep inertia can be substantial, and such measurements as reaction time in those with inertia can be comparable to those of drunk people.
Exercise is a good way to fully wake up. And coffee or other caffeinated beverages do many people a world of good. Drowsy driving is a major public safety hazard and something we at Sleepdex are trying to fight. Do everyone a favor and make sure you are awake before starting the car.
Although daytime naps provide benefits - both cognitive and otherwise - they often leave the napper with sleep inertia upon waking. One challenge for the "power napper" is to avoid inertia. Inertia tends to be worse when naps are taken in the afternoon rather than the morning. A study in Ireland found that executive mental functions are slower to return to normal after a nap than the ability to do simpler things.