Many of us would say that we dream in images, occasionally waking up after a crazy, scary, cinematic adventure in our unconscious heads. There may be a soundtrack too, with voices or music. But what about smells?
Few people would say they smell in their dreams, but Francesca Faruolo talks vividly about her own experiences.
Her dreams are often fragrant, she says.
"I have very positive olfactory dreams, especially featuring orange flower, a flower linked to the heart," says Faruolo, director of the Smell Festival which takes place every May in the north Italian city of Bologna.
"Olfactory dreamers do exist," she insists. "They are people who, in their everyday lives are either very sensitive to smells or have a highly trained sense of smell."
Academics have done little research on the subject, tending to focus instead on the effect of external smells on our dreams.
Some of the earliest documented scientific experiments date back 150 years, long before the advent of sleep research labs and electroencephalograms. In his 1865 work, Le sommeil et les reves (Sleep and Dreams), French scholar and physician, Alfred Maury - whose studies were cited by Freud in The Interpretation of Dreams - describes his self-induced sensory dream experiences.
To ascertain whether olfactory stimulation could affect our dreams, he instructed an assistant to put eau de Cologne under his nose while he was asleep. On awakening, he reported that he had dreamt he was in Cairo, in the workshop of Giovanni Maria Farina, the perfumer who invented cologne, before embarking on an exciting series of adventures.
Nonsense, says Rachel Herz, a professor of psychiatry at Brown University and author of The Scent of Desire. Her research and experiments indicate people do not respond to odours while they are in the dreaming phase of sleep (REM) or deep sleep.
"You cannot smell while you are asleep," she says. "You don't smell the coffee and wake up; rather you wake up and then smell the coffee."
But, she says, if we very briefly wake up and perceive the scent of coffee, it will wake us further if we are interested in it.
Any odours that are experienced in dreams, like Faruolo's, are "created by the brain not from outside".
That is one theory. Prof Thomas Hummel of the University of Dresden's Smell and Taste Clinic has another. His research corroborates Herz's conclusion that smells do not rouse us from sleep, but olfactory stimuli do influence our dreams, he suggests.
In one experiment, in which volunteers were stimulated with hydrogen sulphide (the rotten-egg stink-bomb smell) and phenyl ethyl alcohol (which resembles the smell of roses), participants reported having more positive dreams with the sweet-smelling stimulus and more negative dreams with the foul-smelling one.
However, none of them reported Maury-style direct incorporation of the smell stimulus into their dreams.
Both Herz and Hummel, though, do accept that olfactory dreams exist, as does Rosalia Cavalieri, author of Il naso intelligente (The Intelligent Nose), though she says they are very rare.
One reason for this, she speculates, is that the sense of smell is "disregarded, doomed to marginality", especially in Western culture, which gives priority to sight and hearing. Immanuel Kant, the German philosopher, called smell the "least rewarding and most easily dispensable" of our senses.
"Just like dreams, smells mostly act outside our conscious sphere and nevertheless condition our behaviour," Cavalieri says. "And, just like dreams, smells have an evocative nature which is difficult to express in words."
Olfactory perception studies have shown that if a smell is familiar or can be named, people perceive it better, even when conscious. This would explain why so many olfactory dreamers are, in Faruolo's experience, involved in the perfume sector. They pay more attention to the sense of smell, and are better at describing smells in words.
Faruolo dedicated this year's Smell Festival to "the scent of dreams" partly because she is fascinated by the idea that in dreams it may be possible to experience smells we have never experienced, or that do not exist in reality.
The most convincing evidence of this on record, according to Cavalieri, comes from the memoirs of the deaf and blind author, Helen Keller, "'compelled' to exert" her sense of smell more than most people. In her book, The World I Live In, Keller writes:
"I smell and taste much as in my waking hours... In my dreams I have sensations, odours, tastes, and ideas which I do not remember to have had in reality."
So perhaps, if we develop our odour awareness, we are all capable of having fantastic scent-rich dreams.
A word of caution though from Professor Hummel, who confesses that, in spite of years of research in the field, he still does not experience olfactory dreams.
"I do not have any. But my wife does."