With sleep, I feel like you’re sort of still there since when you wake up you have the perception that time has passed. What’s truly unnerving is going under anesthesia, where it’s like that period of time just never existed at all.
I found the experience strangely comforting - basically the closest approximation to what death is like. One second you're there, and the next you're not. It wasn't bad or scary or painful or anything. It was just existence and then nothingness and then the feeling of coming back online realizing what happened.
And when I was coming back online, my thinking was: Well, I'm glad to be back! And if I hadn't come back, would I have even noticed? (The answer is no).