Answer:

nightmares

Answer	Crossword Clue
NIGHTMARES	What horses keep late hours?
NIGHTMARES	Nocturnal phenomena.
NIGHTMARES	Frightening experiences

NIGHTMARES	Female equines that run after dark?
NIGHTMARES	Scary experiences
NIGHTMARES	Sleep clinic topic
NIGHTMARES	Unpleasant experiences
NIGHTMARES	Nocturnal females?