Excessive sleepiness is not usually a consequence of insomnia but can be a sign that there is insufficient opportunity for sleep, or that sleep is compromised. Movement problems can disrupt sleep, as can sleep disordered breathing. People are more likely to be excessively sleepy at times of day when they would usually be asleep, which can affect safety when driving or operating machinery. Narcolepsy is a condition that is characterised by sleepiness (but most sleepiness is not related to narcolepsy).
Excessive sleepiness is measured in specialist sleep services using various tools including the Epworth Sleepiness Scale (ESS), which is a self report questionnaire. The most accurate measurement of sleepiness can be gained using specialised laboratory based procedures to test objectively how sleepy someone is including the Maintenance of Wakefulness Test (MWT), which tests the person’s ability to stay awake and the Multiple Sleep Latency Test (MSLT), which tests their ability to fall asleep on demand.