I did not listen to the Allen and I paid the price.
No, I don't think I have sleep apnea .... Dozing off in the cafe was, trust me, pure boredom. Getting sleepy doing long long tones is pretty easy to figure out, with all the good advice in this thread.
b) Doing these long tones one after another (with no rest) .... oxygen deprivation
c) CO2 buildup
Cure: Stop doing long tones
Alternate cure: Stop doing them one after another
Alternate cure #2: Adopt a soft volume, slur-filled, long tone alternative