Why don’t we smell when we’re asleep?

And is the same true of animals?

Because of my small bladder, we still have a house.

A few nights ago, I woke up at 3 am as I usually do, but once I was upright and blearily staggering toward the bathroom, I began to notice a sickly-sweet smell, an odor of burning plastic. It was strong enough to wake me up entirely. It seemed to fill the house, stronger at head height than when I’d been laying down.

I woke Andrew and he eventually found the source in our attic, which was filled with smoke but no flame. The connection for the HVAC air handler had been slowly melting, as you can see in the photo above. Only two hairlike copper wires were still connected and keeping the system running.

We now have a new connection and plenty of heat during our current arctic air blast, but it definitely felt like a close call. Andrew is a former firefighter and told me the reason people often die in housefires is because they never smell the smoke – our sense of smell doesn’t work while we sleep. If you don’t have a smoke alarm in your attic (which we now have on order), you might never realize you have a smoldering mess eating away your roof above you until it’s too late.

Is that true? Do humans not smell at night? If so, why does that particular sense shut down? And is it also true of animals? I’ve heard stories of dogs waking up their families to escape a burning house, so does that mean animals (dogs) do keep their sense of smell as they sleep? What about wild animals, who might benefit from smelling predators creeping toward them at night?

Why don’t we smell when we’re asleep?

First, a little bit about how humans sleep:

Source: Sleep Foundation

When you’re asleep, you’re not just comatose until it’s time to wake up. Your brain goes through four distinct stages during a cycle, and you experience 4-6 of those cycles each night. As you drift off to sleep, you spend the first few minutes in the N1 stage, when your mind wanders and then settles into light sleep. If you’re stressed, you might have difficulty moving past this stage as you process your to-do list and that snarky email your boss sent. Almost any sensory input – sounds, touch, or smells – can wake you up when you’re in this N1 stage.

Light sleep, or the N2 stage, is when your breathing slows and your muscles relax. During this stage, you are resistant to being woken up by external stimuli. Altogether, people spend about half their night in this cycle. Deep sleep, or the N3 stage, is that restorative sleep we all seek. This is when your body releases growth hormones to heal sore muscles from that long hike you took and strengthens your immune system. You usually spend more time in N3 during your earlier sleep cycles, and these sections shorten as the night goes on.

The final stage (before you start the cycle all over again) is REM, also known as Rapid Eye Movement. This is when you dream. During this stage your brain wakes up but your body is temporarily paralyzed except for your eyes, which flicker and move beneath your eyelids as you follow your dreams. This stage is viewed as critical to helping your brain retain memories and improve cognitive function. (Interestingly, this study suggests that taking sleeping pills may interfere with your body’s ability to clean up waste during the REM stage.) During a typical night, the earlier sleep cycles contain short REM cycles whereas later in the night you might dream for around an hour. REM makes up about 25% of total sleep time for adults. Older adults spend less time in REM whereas newborns slide immediately into REM sleep.

Photo by Andrea Piacquadio

So, what about smell and sleep?

As you’d suspect, the deeper asleep you are, the harder it is for any sensory stimuli to wake you up. Scents will wake up a person in the early stages of sleep, as will touch or sound. But when sleepers are in the REM (rapid eye movement) or deep sleep stages, it takes a lot to wake someone. That slight nudge that stops your partner from snoring will work in the early stages, but it could take a bigger shove if they’re in REM. However, that doesn’t appear to work the same with scent – adding stronger scent at deeper levels of sleep won’t awaken someone from deep slumber.

In one study, researchers presented sleepers with a pleasant peppermint smell and a harsher scent called pyridine which is derived from coal tar and is actually a byproduct of fires. Neither woke the subjects, but a moderate sound woke them up almost every time. Perhaps that’s because our brain didn’t recognize those as “dangerous” smells? Nope, this study tried introducing artificial smoke into a room and it still didn’t rouse the sleepers.

It appears that the thalamus, an egg-shaped part of your brain about the size of a walnut, serves as a “sensory gate” that quietens the sensory system while you’re asleep. It blocks out noises and touch and visual stimuli up to a point – if the sound is loud enough or the shaking is hard enough, you will wake up. However, that’s not true of smell – people do, it turns out, die in housefires even when smoke is incredibly dense. Why? Researchers aren’t sure, but it seems that the olfactory system is wired differently than the other senses, and may not rely on the thalamus as heavily as the other senses. "As the saying goes," said one study’s co-author Mary A. Carskadon, "we 'wake up and smell the coffee,' not the other way around."

Interestingly, insomnia might be due to impairments with sensory gating. In people who have the type of insomnia that makes it difficult to get to sleep, it appears they have problems with auditory sensory gating. It makes you wonder – would earplugs or a sound machine help with this?

Studies have also found that everyone’s sense of smell is tied to our circadian rhythms and varies throughout the day. Our strongest window of smell may be in early evening, around 9pm, while our weakest is between 3am and 9am.

It seems like the only reason I was able to smell the burning plastic wires a few nights ago was simply because I was at the end of one sleep cycle and about to start another – that vulnerable N1 stage allowed me to wake up enough to listen to my bladder. Once I was awake, my olfactory system kicked in and my brain recognized the smell as something dangerous.

But what about dogs?

Why didn’t Toby wake us up like a heroic Lassie? Well, to be fair, he’s locked out of our bedroom because he’s an incredibly obnoxious snorer for a smallish dog. Perhaps I’m not giving him credit for having tried and given up, unbeknownst to us.

There are lots of stories about dog heroes that saved their families from the flames like Whiskey, a puppy who was rescued after having been shot, probably in an attempt to end his life, but who ended up rescuing his two elderly owners from a nighttime fire. Or Dakota, a deaf dog who alerted his family (two of whom were firefighters!) to a fire that destroyed their home as they were sleeping. He couldn’t hear his own barking, but he was able to smell the smoke and wake his family.

There aren’t many studies looking at animal’s olfactory systems and how they work at night, but it is thought that the human sleep cycle is much the same for other mammals. It’s possible that Whiskey and Dakota and other heroic dogs were, like me, simply in a lighter stage of their sleep cycle and woke up to smoke. Of course, dogs have an incredible sense of smell, with 300 million scent receptors compared to our 6 million. So, it’s very possible they smell burning at a much earlier stage than people would. Anders Hallgren has developed a training method to capitalize on dogs’ remarkable ability to smell. He trains dogs as living smoke alarms to alert families to the presence of smoke before a smoke alarm goes off. Most of his stories, however, are of daytime fire detection.

All in all, it seems like there are several lessons to be learned here: don’t rely on your dog to alert you to a nighttime house fire, buy a smoke alarm for your attic, and appreciate your family members who have tiny bladders.

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Weird Nature:

This week’s Weird Nature comes from close to home. As Andrew was splitting firewood two days ago, check out what he found inside one log:

That’s a bullet, deep inside a tree. Who knows how long it’s been there? This was from a section of the oak that was probably 30-40 feet high. Someone may have been hunting and missed their shot, burying the bullet in the tree. From there, the tree grew around it for years, maybe decades…