It's broader than many assume:
Consistent difficulty in falling or staying asleep that causes problems during the day is all it takes to be an insomniac. That can mean instead of getting, say, 7 hours of sleep each night, one gets only 5 or 6 a handful of nights a week. That can happen, for example, if you devote 7 hours to sleep but it takes you 30 minute to fall asleep and then you wake up in the night and can’t return to sleep for another 30 minutes or more. You’re still getting a lot of sleep, just not quite what you need. The key is that it has to be problematic for daytime function or the cause of distress. There are tons of people who only sleep 5 or 6 (or fewer) hours many nights of every week. But if they’re not upset by it and they function fine, they’re not insomniacs.
The Dish 