The idea that certain scents promote sleep has been popular for thousands of years. Ancient Egyptians believed that myrrh increased sleep quality, for example, while recent studies have suggested that citrus smells may help sleep by improving mood, and a lavender scent is often touted as the perfect pillow spray. However, there has been little systematic objective investigation of the impact of specific odours on sleep, write the authors of a new paper in Scientific Reports. Agnieszka Sabiniewicz at the Technical University of Dresden, Germany and colleagues now report just such a study.

The participants were 139 healthy men and women. They first went into the lab to complete a batch of tests, including measures of their smelling ability and also symptoms of anxiety and depression. They were then given a nose clip to take home with them. Each clip delivered one of: an orange odour; l-Laurinal (a sweet floral scent); a perfume with citrus, floral, musky and orange scents; no scent at all.

Over a two week period, the participants wore their clip each night and completed a sleep diary each morning. (The diary asked about their quantity and quality of sleep.) A subgroup of 66 participants also used wrist-worn devices that measured their minutes of REM, deep sleep and light sleep.

After two weeks, the participants went back into the lab for repeat testing. They also reported on how pleasant or otherwise they had found their scent. Then they went home and, without wearing the nose clip this time, completed a sleep diary each morning for the following two weeks. At the end of this period, they went back into the lab, to re-take the key tests for a final time.

The team found a few small differences in the impact of specific odours on participants’ depression and anxiety scores. Those who’d sniffed the perfume tended to report less anxiety than those who’d smelled the Laurinal, although this effect was only marginally significant. And those who’d sniffed either the perfume or no odour had better depression scores than those who’d sniffed the Laurinal.

But the main aim of the study was to investigate impacts on sleep. The team found that, whatever the nose clips delivered, the participants generally reported feeling better rested as the study period went on. (So even the zero-odour, control group felt that they benefited.) Also, participants who found the odour they had been assigned particularly pleasant tended to report feeling better rested at the end of the study, compared with the beginning.

However, when the team compared the sleep quantity and quality data for each of the four nose clip groups, they found no differences — none of the odour groups had slept any better than the control group. As some earlier studies have suggested that certain odours can improve sleep (the scent of bitter orange has been linked to better sleep quality, for example), why did this research find no odour benefits? One possibility is that perhaps it had something to do with how the scent was delivered— via nose clip, rather than a spray or a scented candle, for example.

Some researchers have suggested that certain scents (including lavender and citrus odours) may act via the central nervous system to exert a direct relaxing or mood-boosting effect, and affect sleep in this way. However, this new work does suggest that how much you like a smell is what matters, rather than the odour itself.