This term describes the nightly experience of those
unfortunate schmucks who just absolutely cannot fucking
fall asleep, no matter how much thrashing around in
bed they do or how much ruminating over past mistakes they indulge in. Instead of falling asleep, an insomniac is simply failing asleep.
Alice: "Wow, you look like hell"
Bob: "<mumble> last night I was failing asleep and couldn't stop thinking about some shit from middle
school when I got my bike stolen"