As said above nodding is the eyelids usually and also another thing with nodding is 'the dreams', not sure if it's just me but when i can get one going (very fucking rare since i only use tramadol n codeine) i kind of go into mad little vivid dreams, not like the type of nasty ones you get on mirtazapine or something just very pleasurable dreams and my breathing is fine, if you're feeling CNS depression the last thing you will be doing is nodding, you will be panicking and finding it even harder to breath, a nod shouldn't make one panic, a nod is a euphoric state including lucid short dreams when i let go, at least for me, your eyes will just keep shutting and feel heavy, your breathing should be normal or only very slightly less.
There's a very big difference between nodding out and nearing death, oxygen deprivation makes you feel terrible not euphoric