You don't need self-heating, or to actually put the thermistor into the nostril. Air coming out of the lungs is at body core temperature, and a thermistor of short enough time constant placed 1/2 inch from the nostril opening will show a very strong fluctuation with each breath, as long as the ambient is lower than 37C. Another thermistor can be in the vicinity but not in the direct flow, to be subtracted. Then, block DC one way or another. I rigged up something like this to test myself for sleep apnea (not differential between nostrils).
There's a famous story about Lev Landau (physicist)--he was sitting in a meeting of the USSR Academy of Sciences listening to someone presenting a paper purporting to show that scientific creativity reached a peak at times of maximum solar activity. Landau leaned across to a colleague and said, "Yes, but this paper was conceived in a deep solar minimum."
"It is a known scientific fact that marijuana increases creativity from eight to eleven times!" - Jack S. Margolis & Richard Clorfene, "A Child's Garden of Grass (a pre-legalization comedy)"
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