Transient disorientation, grogginess and performance impairment that can occur after wakening. The length and intensity of sleep inertia is greatest when the individual has not had enough sleep, is woken from slowwave sleep (non-REM stages 3 and 4) or woken during the WOCL.
Source: Fatigue Management Guide for ANSPs
People tend to feel groggy for a period of time following awakening from a long sleep or nap. This condition of temporary disorientation and reduced performance is called "sleep inertia". It usually lasts from 30 to 60 minutes but can sometimes reach two hours. The duration tends to be longer after sleep deprivation. While the biological reason for the phenomenon has not been determined, some researchers hypothesize that this is a protective mechanism that helps maintain sleep after an unwanted waking.
Sleep inertia is particularly strong early in the morning (4 a.m. to 5 a.m.) because this period coincides with the deeper stages of sleep and the window of circadian low.
Sleep inertia is particularly challenging for shift workers that may need to stay focused at any time of the day. This includes pilots, air traffic controllers and aviation maintenance personnel. It has a range of undesired effects, e.g.:
- Desire to fall back asleep
- Impaired cognitive ability
- Impaired visual attention
- Impaired spatial memory
Caffeine can be used to reduce the effects of sleep inertia. Research has found that taking 100 mg of caffeine on awakening reduced the time of sleep inertia, restoring reaction time more quickly compared to placebo. Another option is to take a caffeine shot just before a short (10-20 min) nap, as caffeine usually takes about 30 minutes to reach full effect. Thus, on awakening the person experiences both the refreshment of the nap and the full effect of caffeine. Note that this is not effective if the nap is longer than 30 minutes.
More generally, good sleep habits help in reducing the effects of sleep inertia. These include:
- moderate caffeine intake (too much of the substance can cause sleep issues and thus increase the length of sleep inertia).
- avoiding the accumulation of sleep debt (i.e. getting more than the usual sleep after e.g. a night shift).
- appropriate room temperature (if it is too high, the body cannot cool before sleep).
- ambient light adjustment (exposure to artificial light in general and light coming from outside the bedroom may reduce sleep quality).