Study participants easily detected odors when awake and in the early transition into sleep (Stage One sleep) but, once asleep, did not. The findings indicate a significant alteration of perceptual processing as a function of sleep.
"Human olfaction appears insufficiently sensitive and reliable to act as a sentinel system," said Rachel S. Herz, visiting assistant professor of psychology and an author of a study titled "Minimal Olfactory Perception During Sleep: Why Odor Alarms Will Not Work for Humans," published in a recent issue of the journal Sleep.
Researchers studied the effects of two scents the pleasurable peppermint and offensive pyridine on six participants in their early 20s.
Over two nights, participants wore an elastic chinstrap to encourage nose breathing. Researchers presented odors through a tube attached to an air-dilution device. The odors were tested during moderately deep Stage Two sleep, deep Stage Four sleep and REM (rapid eye movement) sleep.
No one responded to peppermint during sleep. Responses to the intense and noxious pyridine were infrequent and did not wake any participants in the deepest stage of sleep. Pyridine is a component of coal tar and used as a herbicide for firewood, and thus a likely byproduct of many real fires, according to the authors.
However, sound woke the participants regardless of the sleep stage. A moderately loud auditory tone produced arousal from sleep virtually every time the scents did not.
Most odors stimulate people's trigeminal nerve to some degree, which is relevant to detection of the scent. Both odors used in the study were of comparable trigeminal strength even though one was pleasant and the other aversive at high concentrations.