Ever wonder how blind people dream? Though some can dream visually, most of them use their other senses, namely hearing and touch.

A new study, published in the journal Sleep Medicine, shows that being blind certainly alters how one dreams. For the study, researchers observed 11 congenially blind, 14 late blind, and 25 sighted control participants over the course of four weeks. Every morning, participants completed surveys in relation to sensory construction of the dream, its emotional and thematic content, and the possible occurrence of nightmares. Scientists also tested participants’ ability to produce visual images during waking cognition, sleep quality, and depression and anxiety levels.

Overall, the blind heard sounds and voices more than they saw anything in their dreams. The late-blind group, however, did experience some visual dreaming. All blind participants were also about four times more likely to have nightmares. Although the scientists didn't know why, they concluded that the blind might feel more threatened than other non-blind people.

According to the Centers For Disease Control and Prevention, more than 3.3 million Americans, ages 40 and up, are legally blind or have low vision. Some of the leading causes of blindness include cataracts, diabetes, and glaucoma, which can all contribute to poor vision. Cataracts develops when the eye's lens gets cloudy. About 20.5 million Americans in the same age group have a cataract in one or both eyes. Glaucoma, on the other hand, is a disease that can damage the eye's optic nerve and result in vision loss. Diabetic retinopathy, a common complication of diabetes can also cause you to lose your sight.