I learned a new term: Survivor's Euphoria. Only having had relatively minor procedures, I have only had relatively minor instances. But I have had a feeling of "I came back" which I have solely after waking up from anasthaesia. As if the interrupted mental processes carry some flow state forward, which I re-attach to.

There's a longer baseline term which might go with this: Survivor's Depression. I have found after successful surgery, diagnostics, any kind of procedure after the initial elation, I have a very strong down-mood. It's not unlike coming back from holiday and feeling exhausted.