45. Olfaction and Sleep

As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy. Coming from William C. Dement, one of the pioneers of contemporary sleep research, this statement depicts sleep as a neurobiological black box. One of the best ways to probe such black boxes is through exceptions, and olfaction stands out as such an exceptional sensory system during sleep. Specifically, whereas sensory stimuli presented during sleep typically wake, this is not the case with odors. In fact, odors may promote sleep. In turn, they remain processed by the sleeping brain, and provide a telling window onto sleeping brain capabilities. Here, we briefly review the foundations of sleep, and then extensively detail the literature on olfaction in sleep, concentrating on studies in humans. We speculate that the unique interplay of sleep and smell whereby odors are processed in sleep without causing wake reflects unique aspects of olfactory neurophysiology, particularly the direct projections from periphery to cortex without a thalamic relay. Finally, although the mechanisms allowing odor processing during sleep without arousal remain unclear, this phenomenon lends itself to using olfaction as a window onto sleep mentation. This approach has uncovered several aspects of learning and memory during sleep. We review these efforts, and conclude with detailing their potential application in the treatment of disease.