Abstract
Mental activity in sleep often involves visual and auditory content. Chemosensory (olfactory and gustatory) experiences are less common and underexplored. The aim of the study was to identify olfaction-related factors that may affect the occurrence of chemosensory dream content. Specifically, we investigated the effects of all-night exposure to an ambient odour, participants’ appraisal of their current olfactory environment, their general propensity to notice odours and act on them (i.e., odour awareness), and their olfactory acuity. Sixty pre-screened healthy young adults underwent olfactory assessment, completed a measure of odour awareness, and spent three nights in weekly intervals in a sleep laboratory. The purpose of the first visit was to adapt to the experimental setting. On the second visit, half of them were exposed to the smell of vanillin or thioglycolic acid and the other half to an odourless control condition. On the third visit, they received control or stimulation in a balanced order. On each visit, data were collected twice: once from the first rapid eye movement (REM) stage that occurred after 3 a.m., and then shortly before getting up, usually from a non-REM stage. Participants were asked to report the presence of sensory dream content and to assess their current olfactory environment. Neither exposure, nor participants’ assessments of the ambient odour, or olfactory acuity affected reports of chemosensory dream content but they were more frequent in individuals with greater odour awareness. This finding may have implications for treatment when such experiences become unwanted or bothersome.
Highlights
Sleep, which manifests behaviourally as being perceptually disengaged and unresponsive to the environment, may seem to observers to be a more or less uniform state of inactivity
The participants were 60 healthy young adults (37 males, mean age 23.43 ± 3.80 SD years, range 19–35 years) who had been pre-screened with a set of questionnaires and inventories for the absence of (i) psychiatric, neurological, and other conditions influencing the sense of smell [44] or sleep [45,46], (ii) problems in daily life due to olfactory impairment [47,48], (iii) mild or more severe depressive tendencies [49], (iv) tobacco dependence beyond light smoking [50], and (v) alcohol dependence beyond moderate drinking [51]
The exclusion criteria were as follows: (i) any current psychiatric, neurological, or other condition that might affect olfaction or sleep, or a history thereof, (ii) a score below a cut-off point of 38.5 on the Questionnaire of Olfactory Disorders–Negative Statements (QOD-NS) that would be potentially indicative of absence of normosmia [52], (iii) a score on the Beck Depression Inventory II (BDI-II; [49,53]) above the minimal range of 0–13 points [54], (iv) smoking over 90 cigarettes a month, (v) drinking more than one or two drinks a day in women and men, respectively, where a ‘drink’ is 0.33 L of beer, 0.2 L of wine, or 0.02 L of liquor, (vi) history of substance abuse, illicit drug use, (vii) pregnancy or breastfeeding in women
Summary
Sleep, which manifests behaviourally as being perceptually disengaged and unresponsive to the environment, may seem to observers to be a more or less uniform state of inactivity. We experience mental elaboration that is not directly observable and is only reported upon awakening in a different physiological state. MSE content referred to as ‘dreamlike’ is rich in (predominantly visual) imagery, tends to be bizarre, hallucinatory, emotionally charged, and have a complex story-like narrative organisation. It became associated with the rapid eye movement (REM) sleep stage [3,4,5] and is viewed by some researchers as synonymous with ‘dreaming’ [5]
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