Nightmares are vividly realistic, disturbing dreams that rattle you awake from a deep sleep. They often set your heart pounding from fear. Nightmares tend to occur most often during rapid eye movement (REM) sleep, when most dreaming takes place. Because periods of REM sleep become progressively longer as the night progresses, you may find you experience nightmares most often in the early morning hours.
A Night Terror is a partial waking from sleep with behaviors such as screaming, kicking, panic, sleep walking, thrashing, or mumbling. They are harmless and each episode will end in deep sleep.
Hypnagogia is the experience of the transitional state from wakefulness to sleep: the hypnagogic state of consciousness.