sleep apnea and other sleep-related breathing disorders, narcolepsy, idiopathic hypersomnia, psychiatric disorders, chronic use of drugs or alcohol, medical disorders like an overactive thyroid, insufficient
sleep, neurological
disorders,
sleep-
wake cycle circadian rhythm
disorders (eg. nightshift work), etc.
“Onset insomnia” is a condition where a person faces difficulty in falling asleep at the beginning of night, when he goes to bed.