Smell is a powerful thing. It helps us trigger memories and attract potential mates; it’s the only sense to have a direct connection to our brains (through the olfactory nerve) and we humans have the ability to detect roughly one trillion scents. Based off someone’s sense of smell alone a doctor could plausibly predict gender, race, illness and, in older adults, mortality.
Your sense of smell, however, can’t be relied on to wake you up. Much has been written about different scents' ability to conjure certain emotions, and everyone thinks they have been roused by the smell of sizzling bacon in the morning. But your brain is playing a trick on you: scent alone won’t wake; audible alerts are necessary for timed risings and fire alarms.
In a 2004 study from Brown University, researchers pumped peppermint and pyridine (a common byproduct of fire, bearing a distinctly sour odor) into the rooms of six dozing subjects. The subjects were placed in a position that encouraged nose breathing and, over the course of two nights, the scents were diffused during various sleep stages. No subjects responded to the peppermint, and the pyridine elicited only a few reactions, none of which occurred during deep sleep.
According to Rachel S. Herz, Ph.D., study co-lead and author of The Scent of Desire, “Only if the sleeper has coincidentally had a micro arousal or is in a very light stage of sleep when the scent starts being pumped into the surrounding air will it be effective.” She adds that scent certainly wouldn't be useful for precise or early wake-up call. “In general, we worry that scent based alerts will not be effective, especially when replacing sound in fire alarms.”
In a similar study performed by the Irondale Fire and Rescue Service in Irondale, Alabama researchers pumped a variety of scents, including citrus and smoke byproducts, into the rooms of 10 sleepers to prove the importance of smoke alarms. Only two subjects were woken up by the smell of fire.
So, check your fire alarms. And know that when you’ve been woken up by bacon in the morning, it's likely the sound of the sizzle that’s roused you — the smell is just a bonus.