Familiar smells 'can trigger nightmares', study claims

Familiar smells ‘can trigger nightmares’ because of a connection between parts of the brain which control emotions and smells, study claims

• In lab conditions, 14 people slept while scientists pumped in the smell of roses
• Participants were woken from their slumber and asked to rate their dreams
• Those who said the smell of roses was familiar to them had worse dreams  
• Experts say areas linked to smells and negative emotions are close together 

It’s not just eating cheese that gives you nightmares – familiar smells can also trigger scary dreams, scientists claim.

A study done in Japan suggested that familiar odours, such as flowers, a candle or fresh laundry, triggered negative emotions in people’s dreams.

Fourteen people slept in lab conditions while scientists pumped the smell of roses into the room. 

The people were all then woken from their slumber throughout the night and asked to rate and describe how negative their dream was. 

Those who had previously said the smell of roses was very familiar to them reported having the worst dreams when they could smell it during their sleep.

This may be because the part of the brain that processes smell is connected to the part which processes negative emotions, including fear, the researchers said.  

Researchers pumped the smell of roses into a room while people were sleeping and found those who had previously said the scent was very familiar to them were more likely to have bad dreams because of it (stock image)

The latest study, published in the journal Sleep Medicine, was conducted at a sleep laboratory at Hiroshima University.

A group of undergraduate students were presented with a rose-smelling mixture and rated how familiar they found the smell, on a scale of one to nine. 

Seven of the 14 people involved rated the rose smell as very familiar, and seven did not.

The participants, who had an average age of 18, went to the lab just before their bed time and settled into a sleep chamber. 

A group of scientists led by graduate student, Satomi Okabe, monitored their sleep and waited until they were in rapid eye movement (REM) sleep.

HOW CAN SMELL CHANGE OUR DREAMS?

Dreams are believed to have crucial clinical significance, the Japanese researchers said.

Studies have indicated that nightmares are correlated with several mental illnesses, including depression, attempted suicide, insomnia and PTSD.

Sensory stimulation, such as smell and touch, can have some effect on dreams. Some stimuli disturb sleep while others can induce it.

Smell is believed to play a large role in how dreams play out because the brain processes it even as we sleep.

The olfactory bulb is the first part of the brain that receives information about smell from the nose.

It sends the information to various parts of the brain which process what the smell is.

During sleep, it is possible that some parts of the brain that process smell are switched on, while others are switched off.

The amygdala, which mainly processes negative emotions, such as fear, is active while we sleep, and therefore may set a negative tone to a dream if a smell is present.

The olfactory bulb also has direct connections to the hippocampus, a part of the brain which processes memory, suggesting that odors might induce a strong recall of emotional memories. 

REM is a stage of deep sleep which occurs several times a night and is the period when dreams occur. Each episode gets progressively longer, starting at around 10 minutes and lasting for up to an hour. 

When monitoring equipment showed participants in the study were in their second REM sleep episode, the smell of roses was pumped through an air vent into the sleep chamber for around ten seconds.

An experimenter woke up the participants just a minute after the smell was stopped – which is enough time for a dream to occur.

The participants were asked to fill out a questionnaire about the dream and rate how positive or negative it was.

If the participant didn’t have any dreams, the experiment was repeated in the next episode of REM sleep.

This also happened in a control condition, whereby an odourless gas was pumped into the sleep chamber. 

The findings show those who had previously rated the rose smell as familiar reported having a higher frequency of negative dream emotions during REM sleep. 

The researchers said a familiar smell is perceived as stronger, and therefore recognised by the brain even during sleep. 

And they said a close connection between the part of the brain which controls smells and the part which controls emotions may be to blame for the well-known smells causing bad feelings. 

The olfactory bulb is the first part of the brain that receives information about smell from the nose.

It sends the information to various parts of the brain which process what a smell is.

However, during sleep, not all of these areas are ‘switched on’ and able to process smell, research shows.

The amygdala, which mainly processes negative emotions such as fear, is one part of the brain which is ‘switched on’ during sleep. It is connected to the olfactory bulb.

Consequently, a familiar smell might induce a strong, negative emotion because only the amygdala is ‘awake’ to receive it. 

Furthermore, the olfactory bulb has direct connections to the hippocampus, which is involved in memory.

Odours during the night might induce a strong recall of emotional memories, the scientists said. 

The study showed that preference of smell did not effect dreams; in other words, a participant may like the smell of roses, but if they are not familiar with it, they would not have nightmares.  

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