The smells that waft under your nose as you sleep may influence what you dream about, say researchers reporting at the American Academy of Otolaryngology Annual Meeting in Chicago.
Scientists from Germany studied the effects of different smells on 15 sleeping female volunteers. The participants were allowed drift into sleep and as they entered the REM phase – the period most associated with dreaming – the strong odour of either rotten eggs or sweet roses was wafted under their noses. A minute later the subjects were woken and asked about the nature of their dreams and how they felt.
"We were able to stimulate the sleeper with high concentrations of positively and negatively smelling odours and measure if the stimuli were incorporated into the dreams and changed the emotional tone of dreams," said lead author, Dr Boris Stuck 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."
The team found that when the unpleasant smell was used the emotional content of the dream was mostly negative, while the scent of roses coloured the dreams with a positive glow.
The researchers claim that this is the first valid study to look at the impact on smells on dreams. They hope that their research may lead to treatment options using odours for people who suffer from sleep problems such as nightmares.
"All our other senses, apart from smell, shut down when we sleep," says Professor Tim Jacob, an expert in smell and taste, from Cardiff University. "Our sense of smell has direct access to the subconscious areas of the brain which affects emotion, mood and memory. So it makes complete sense that different smells can influence the emotional tone of a dream."
The German researchers found that the type of smell did not affect the length of dream or specific content but was able to colour the dream in a good or bad way.
Previous research has found that using smells during sleep can have a powerful effect on memory. Another group of German scientists used the scent of roses on volunteers as they studied and later as they slept and found it improved their performance on a memory test by almost 15 per cent.
"Studies have been done to try and find out which sense has the most power to evoke memory and emotion," says Tim Jacob. "Music can have a powerful effect, but smell has been found to be the most potent stimulus for evoking memory and the emotions associated with it."