have just taken a SIBO breath test. First hand observed the rise of breath hydrogen and methane. The biological machinery is that [bad overgrown] bacteria in the gut produce the gas with some of the gas making into the blood and after that into the lungs. That rise as usual coincided with small attack of coughing, running nose and a bit of anxiety and loss of concentration (was trying to work during the 3hr test). When it happens at night, it does makes falling to sleep harder. Anyway, passed the test with flying colors :) - hit 78ppm hydrogen and 29ppm methane levels (at 40 and 60 minute marks, ie. before colonic response, etc) where 20ppm of either alone is already enough for SIBO diagnosis. So it isn't just imagination.