I have a strong connection in my head between "fingers not working" and "danger, brain not right" . It has saved me several times. Good thing is, its a simple rule that works for hypoxia, hypothermia, narcosis etc. If you struggle with something like a buckle, keys or zipper, stop.
It happened to me skydiving. That time was hypoxia, but I find the two very similar. There was traffic passing below us, so the pilot held and spiraled up as we waited. When it was time to exit I couldn't tighten my chest strap and stow the extra strap. I told my group something was wrong and I was riding the plane down. At about 8000 feet my periferal vision came back and I could suddenly see in color again. Until it did I had zero awareness that I was seeing only a tiny tunnel of black and white.
The few times with hypothermia I was also only keyed in by clumsy fingers, never by shivering nor feeling particularly cold or odd.
Trying to rapidly tap your thumb to your pinky is a good test. Don't depend on being aware enough to recognise it in yourself, but for me "fingers not working" has the best chance of penetrating a foggy brain.
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