Most of my dreams are boring. I’m typically talking to someone I know, either on the phone or in person, in a familiar place. The conversations tend to include ideas that I’ve read about or experienced recently in my waking life. Some of my dreams don’t even have the intrigue of another character; I’m just alone, sitting in front of my computer, reading or Tweeting or whatever. Occasionally when on deadline I’ll have a classic anxiety dream, but even that’s pretty tame, with me flunking out of college because I forgot to go to class.

No matter what the psychological message, my dreams are almost entirely visual and auditory. Every once in awhile I’ll have a dream that includes a physical sensation, like my stomach dropping as I ride down an elevator. But it’s so rare that when it happens the feeling sticks with me all day. That probably makes sense, as I rely on my vision and hearing far more than other senses. But what about people who lose their sight, or could never see? What do they dream of?

A group of Danish researchers posed those questions in this month’s issue of Sleep Medicine. They recruited 50 adults: 11 blind from birth, 14 who became blind sometime after age 1, and 25 non-blind controls. Participants agreed that for a period of four weeks, whenever they had a dream they would fill out a computer questionnaire about it as soon as they woke up. (The blind volunteers used text-to-speech software.)

The questionnaire asked about several aspects of the dream: the sensory impressions (Did you see anything? If so, was it in color? Did you taste? Smell? Feel pain?); the emotional content (Were you angry? Sad? Afraid?); and the thematic content (Did you interact with someone? Did you fail at something? Was it realistic, or bizarre?). The questionnaire also asked whether the dream was a nightmare.

All of the non-blind control participants reported a visual impression in at least one dream. In contrast, none of the participants who had been totally blind since birth did. For the group with later-onset blindness, the longer they had lived without sight, the less they saw in their dreams.

Just as there are many ways to take in the world, there are many ways to dream about it. Blind people dream, just as they live, with a rich mix of sensory information.

About 18 percent of the blind participants (both congenital and later-onset) reported tasting in at least one dream, compared with 7 percent of controls. Nearly 30 percent of the blind reported smelling in at least one dream, compared with 15 percent of controls. Almost 70 percent of the blind reported a touch sensation, compared to 45 percent of controls. And 86 percent of the blind reported hearing, compared with 64 percent of controls.

(The differences are more drastic when looking only at the congenitally blind group. Among these participants, 26 percent tasted, 40 percent smelled, 67 percent touched and 93 percent heard in at least one dream.)

Despite these sensory differences, the emotional and thematic content of dreams isn’t much different in the blind and the sighted. Both groups reported about the same number of social interactions, successes, and failures in their dreams. They had the same distribution of emotions, and the same level of bizarreness.

There was, however, one notable difference between the dreams of the congenitally blind and controls. The blind had a lot more nightmares: around 25 percent, compared with just 7 percent of the later-onset blind group and 6 percent of controls. This difference held even after the researchers controlled for sleep quality, which is generally poorer among the blind.

So what could explain all of those nightmares? The researchers don’t know, but they speculated that it may have to do with evolutionary theories about why nightmares exist. “According to these theories, nightmares can be seen as threat simulations, as a mentally harmless way by which the human mind can adapt to the threats of life,” the researchers write. “The nightmare gives an individual an opportunity to rehearse the threat perception and the avoidance of coping with the threat.”

This seems to agree with what the congenitally blind participants reported in the study. Their nightmares included events such as getting lost, being hit by a car, falling into manholes, and losing their guide dog — all very real threats in their waking lives.