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My question is whether our sense of smell is stronger when we sleep? From an evolutionary perspective this could have been beneficial in terms of runaway camp fires, marauding saber tooth tigers or the testosterone of night-raiding cave men.

Does this effect back off when we wake, as our eyesight comes into play? And, if so, how do we account for the claims that many people die in night time house fires before they wake up?

I'm aware that the brain is able to desensitise itself to certain smells and go about its olfactory business elsewhere. That second sniff of fresh-ground coffee beans is never as good as the first. I'd say that most people assume they are 'a bit slow' first thing in the morning until they 'wake up properly', but is this true?

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    $\begingroup$ Of course being lying down and at rest many senses may become acute, in addition that position of the nose is better for registration than fully vertical (watch when your dog smelling distant scents), also if it is something threatening or annoying you will be more sensitized to those stimuli, finally, as with the ear, there is a greater sensitivity to environmental changes during this period of "defenselessness". $\endgroup$ – hexadecimal Jun 22 '17 at 23:00
  • $\begingroup$ From my experience, I disagree with you. My sense of smell and to a smaller degree hearing too, are much more heightened at night. To the point it DOES wake me up. Smells especially, wake me up constantly. Anything unpleasant to me will wake me up quickly. Anything very strong. And if I’m rly tired and sleeping very deeply I’ll start having bad dreams if I don’t wake up quickly from a strong smell. For holidays, if I have to cook something through the night, I know, don’t even try to attempt sleep. Sometimes, I even smell things at night no one else seems to. Weird, huh? $\endgroup$ – Lea Jan 24 '20 at 12:28
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Short answer
The sense of smell may still be operational during sleep, but it is not enhanced. Olfactory stimuli typically will not wake you up during sleep, however.

Background
Olfaction and sleep are an interesting lot. Most sensory stimuli presented during sleep typically cause awakening, especially when when they are intense and sudden (e.g. a loud noise). This is not the case with odors, and even holds for unpleasant ones (Carskadon & Herz, 2004).

In addition, while most sensory systems stop processing their input altogether, odors remain processed by the sleeping brain. This may, at least in part, be due to the direct olfactory projections from the periphery to the cortex, without a thalamic relay being used as with the other senses (Perl et al., 2017). However, they will typically not wake a person up.

The neural adaptation (reduction of sensitivity over time) of the olfactory system is related to repeated or sustained exposure to scents (Dalton, 2000), and it is not affected by sleep per se as far as I know. Hence, taken together these findings support the claim that odor alarms will not work.

Note, however, that fire comes with heat and irritant smoke that will work on the heat and pain receptors in the skin, respectively. These may wake you up, barred the dulled sensory systems pick the stimuli up. And even then it may already be too late to be of any help.

References
- Dalton, Chem Senses (2000); 25(4): 487-92
- Carskadon & Herz, Sleep (2004); 27(3): 402-5
- Perl et al, Springer Handbook of Odor: 111-2

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  • $\begingroup$ That is interesting. Does that mean that the smell of a leak from domestic gas supplies or other "danger smells" which don't produce heat may not wake someone up either? $\endgroup$ – Chris Rogers Jul 19 '17 at 11:38

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