Difficulty drifting off at the start of the night—called sleep onset insomnia—is probably what many people think of when they hear the word insomnia. And if you often wake up much earlier than you need to and can’t fall back asleep, you may have early morning awakening insomnia (according to the Sleep Foundation, this is sometimes considered a subset of sleep maintenance insomnia rather than its own category). Anything less than that is usually considered acute insomnia (also called short-term insomnia or adjustment insomnia).