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.