The research, conducted at the school's Sleep and Depression Lab, involved a small sample of college students. Nearly half of the 93 participants who reported having poor sleep also reported a fear of the dark. They were also more easily startled in the dark than the so-called good sleepers.

"We assume that poor sleepers become tense when the lights go out because they associate the bed with being unable to sleep," said the study's lead author, Taryn Moss. "Now we're wondering how many people actually have an active and untreated phobia."

The findings mean conventional insomnia treatments may have to be modified to take the phobia into account. For instance, insomniacs are often advised to leave the dark bedroom and go into a lit room, but this would not be the way to treat someone who is afraid of the dark.

"A lot more research is needed, but we believe we have stumbled across an unmet treatment need for some poor sleepers," said Colleen Carney, director of the Sleep and Depression Lab.

Carney and her team shared their findings Monday at the 26th annual meeting of the Associated Professional Sleep Societies in Boston.