- Synesthesia is a condition that involves atypical binding between two seemingly independent sensory modalities. The neural origin of this condition is controversial: While some claim that synesthesia originates from stable anatomical irregularities, others suggest that its occurrence relies on functional properties of brain dynamics. In the current study, we aimed to better understand synesthesia by comparing it with another state in which perception is dramatically altered, namely, sleeping and dreaming. Specifically, on the phenomenological level, both synesthesia and sleeping mentation are characterized by hyper-associated mental images. Therefore, in the current study we tested whether non-synesthetic subjects would report synesthetic-like experiences upon awakening (i.e., during sleep inertia). In two studies, healthy subjects were asked to complete a questionnaire in which they rated the extent of their agreement with statements representing synesthetes' cross-sensory experience (e.g., “musical tones trigger the experience of colors”), along with other (masking) statements. Subjects in the experimental group completed the questionnaire during sleep inertia, while subjects in the control group completed the questionnaire during wakefulness. We found that subjects who completed the questionnaire upon awakening from sleep reported greater agreement with the cross-modal statements compared with wakeful controls, whereas no difference was found for the masking statements. Our findings support the functional, rather than the anatomical, view of synesthesia; specifically, synesthetic experience might be the result of reduced cortical inhibition. Moreover, the current study suggests an exciting new research direction, i.e., exploring possible similarities between sleeping and synesthetic brains.