SUNDAY, Sept. 21, 2008 (HealthDay News) -- Can the smell of rotten eggs or roses change the type of dream you have?

Quite possibly, new research suggests.

German researchers are reporting that when people smelled the scent of rotten eggs while sleeping, the nature of their dreams turned decidedly negative, while those who got a whiff of the scent of roses had more positive dreams.

"We were able to stimulate the sleeper with high concentrations of positively and negatively smelling odors and measure if the stimuli were incorporated into the dreams and changed the emotional tone of dreams," said the study's lead author, Dr. Boris Stuck, a professor of otorhinolaryngology at Heidelberg University.

"We found that the sleeper hardly ever dreamed of smelling something. Nevertheless, the emotional tone of the dream did change depending on the stimulation," he said.

Stuck said that previous research had shown that other types of stimulation, such as sound, pressure or vibration, could influence the content and the emotional tone of dreams.

The difficulty in conducting such research, he said, is finding the point where you can introduce a stimulus that's strong enough to influence a dream, but not so strong that it wakes the sleeper. Certain odors, such as peppermint, not only stimulate the sense of smell, but can irritate the nasal passages as well.

To overcome this, Stuck and his colleagues used chemicals that simulated either the smell of roses or the smell of rotten eggs.

The study included 15 young, healthy females. As the women entered rapid-eye movement (REM) sleep, when dreaming occurs, they were exposed to either a non-odorous control smell, the rose smell or the rotten egg smell. Each woman underwent three REM "awakenings," so they were exposed to each test once.

Once awakened, they were asked to report the content of their dreams. In 40 of the 45 awakenings, dreams were reported.

The researchers asked the women to assess the content of their dream on an emotional "coloration" scale that measured the tone of their dream. They were asked to rate the positive or negative coloration of their dream on a scale of 0 to 3. Zero was no coloration and 3 was strong coloration.

After the control stimulation, there was a slightly positive average coloration of 0.5; after the rotten egg smell, the coloration averaged -0.4; and after the rose smell, the coloration was +1.2, according to the study.

"When stimulating the subject with a positive smell, the emotional coloration was positive in nearly every case, while with negative stimulation, the emotional tone was shifted to negative," Stuck said.

The findings were expected to be presented Sept. 21 at the American Academy of Otolaryngology -- Head and Neck Surgery Foundation annual meeting, in Chicago.

Pamela Dalton is an expert on odor perception and a sensory psychologist at the Monell Chemical Senses Center in Philadelphia. She said, "We are aware at some level of our odorous ambient environment at all times, and I don't think we appreciate that. At some level, our brains are always aware. If this study shows that we can alter the emotional content of dreams, think about what an odor can do to your mood without you even being aware."

Dalton said the study findings could be a first step in finding a way to change people's perceptions of emotionally disturbing places, such as hospitals or nursing homes. But, she added, it's also possible that a reverse association could occur, and people might simply begin linking the good smell with the bad place.

"Odors become associated with good and bad very readily," she said.

More information

Learn more about your sense of smell from the Howard Hughes Medical Institute.

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