Exposure of Healthy Individuals to Pleasant Odours During Sleep: Effects on Odour Identification and Sleep Quality

ABSTRACT Odours are closely linked to sleep quality, yet the effects of nocturnal odour exposure on objectively measured sleep remain unclear. This study investigated the impact of pleasant odours on sleep compared with an odourless control and explored differences between similar vanilla‐rich odours with added components. Eighty‐eight healthy participants with subjective normal olfactory function were randomly assigned to one of four groups: Vanilla ( n = 23, mean age ± SD = 26 ± 6 years), floral ( n = 25, mean age ± SD = 26 ± 5 years), sandalwood ( n = 21, mean age ± SD = 26 ± 5 years), or control ( n = 19, mean age ± SD = 26 ± 4 years). Following baseline assessments in the clinic, sleep was monitored at home using a portable EEG and questionnaires. After two odour‐free nights as baseline, participants slept with their assigned diffuser for two weeks (Phase 1), with EEG recordings on two odour‐exposure nights. In Phase 2, participants slept without odour for one week, with a final EEG night recording and follow‐up assessments. Subjectively, odours reduced waking time during the night. EEG data did not demonstrate robust interaction effects of odour exposure on sleep architecture. However, exploratory findings indicated that individuals with poorer baseline sleep quality had greater benefit from odour exposure compared to baseline. Participants exposed to sandalwood performed better in cued odour identification at follow‐up, though without a significant interaction effect. Exposure to pleasant odours during sleep showed potential to enhance odour identification and reduce awakenings, but further studies are needed to confirm these effects. While the overall sleep quality in healthy individuals remained largely unchanged, nocturnal olfactory stimulation shows promise for supporting those with suboptimal baseline sleep.