I'm suffering from this right now. I start to relax, then all of a sudden I feel myself falling asleep which jolts me awake. My heart starts to beat faster and I feel on edge. I also make a conscious effort to breath deeply which makes me breathless. When all this is happening it genuinely feels like your time is up. I laugh, though, because I got up for a pee, and when I came back to bed, my wee puppy was sitting up looking at me which made me smile, and as I type this message I feel completely normal and fine. No horrible feelings, no breathlessness, nothing.
Anxiety is a strange one!!!