I spent months underwater on a US submarine and felt the same thing. Later the Navy pdoc labelled it dissociation, which is in the DSM.

    Common Types and Symptoms

    Depersonalization: Feeling detached from your own body, mind, or actions, like watching yourself in a movie.

    Derealization: Feeling that the outside world is fake, dreamlike, or foggy.

    Dissociative Amnesia: Forgetting important personal information or blocks of time that go beyond normal memory loss.

    Identity Confusion or Alteration: Feeling unsure of who you are or experiencing shifts in your personality.

    Causes and Triggers

    Trauma and Stress: Acts as a defense mechanism against overwhelming stress, abuse, or frightening events.

    Everyday Triggers: Loud noises, strong emotions, or reminders of past trauma.

    Mild States: Getting lost in a book, daydreaming, or "highway hypnosis" while driving.

Being an astronaut seems cognitively expensive. Your time is regimented down to the minute. Your space is regimented down to the inch. The things you bring up and down, including yourself, is measured to the pound. I would imagine you have to constantly be evaluating, assessing, and thinking critically about everything you do all the time. And then to suddenly not have to do that and just live in the moment has to be a massive readjustment.

The mind works with benchmarks on a constant basis. Everything is a comparison - to your prior experiences, other people's experiences, your identity, etc etc etc

Introducing a new benchmark creates a new perspective to measure old perspectives against. In essence, de-sycing your perceptions of self, reality, etc until the new and old normalize out.

(You see this with psychedelics a lot which is why it can be such a valuable therapy for depression and PTSD)

It is no wonder that being put in a giant space bullet, shot into the darkness of space, with the light of your world reflected back at you like the ultimate out of body experience leads to an "observer" sensation.

I thought this was going to be about feeling watched, reminding me of philip k dick but it's more like being john malkovich which might be scarier

This article recalls the Steven Wright joke:

“I woke up one morning, and all of my stuff had been stolen and replaced by exact duplicates.

I couldn't believe it.

I called my roommate in and said, 'Come here, look at this stuff, it's all an exact replica. What do you think?'

He said, 'Do I know you?'”