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"