Mixed sleep–wake states
Mixed and transitional states are very variable, but can be categorized according to the state in which they arise.
Wakefulness
Sleep inertia is the drowsiness, often associated with physical incoordination and occasionally with confusion, that is present after waking from sleep. It may be the result of delayed activation of the prefrontal cortex at the transition from sleep to wakefulness. If it is severe it may cause a confusional arousal in which lack of awareness is combined with often complex motor behaviour.
Sleep inertia often lasts for 30–120min after waking in the morning. It almost always arises when waking occurs from NREM rather than REM sleep, particularly stages 3 and 4. It is commonest after oversleeping, after daytime naps if stages 3 and 4 NREM sleep are entered, which is usual if naps last for more than 30 min, and in idiopathic hypersomnia. Sleep inertia can be shortened by caffeine, suggesting that it may be related to an increased influence of adenosine after the onset of wakefulness.
Nature of Sleep and its Disorders
Aspects of REM sleep can also intrude into wakefulness. The ability to recall dreams on awakening is a common example of this, and the hallucinations associated with delirium, such as delirium tremens, probably represent partial REM sleep intrusion into wakefulness. The loss of muscle tone with laughter (cataplexy) is an example of pathological REM sleep intrusion into wakefulness. Sleep-onset sleep paralysis may also be regarded as intrusion of a fragment of REM sleep into wakefulness.