I've had the same experience when sleep-deprived (which is really the only condition in which I can nap). I'll grab a couch at work and recline and close my eyes. What follows is perceived as a relatively brief period -- twenty minutes or so -- of being somewhat abstracted from reality, with my mind operating in that semi-dreamlike state you sometimes get when waking up slowly. Then I'll open my eyes, slightly fuzzy but less tired, and go back to my desk to find that 60-90 minutes have passed. (This is almost always in the context of work-related sleep deprivation, like being in a crunch on a project or up late because of a crisis, so I've never gotten any guff over it; in any case, it happens very rarely.)

What's interesting is that I definitely feel like I'm perceiving the world with some continuity during that time -- I'll hear coworkers' conversations, people walking around, etc. -- but apparently I miss things like people coming up and checking if I'm awake, or even the time someone laid my hoodie over me as an improvised blanket.

If I sleep any longer than 90 minutes or so, I just perceive it the same way I do sleeping at night.

The brain is a strange contraption, isn't it?

When I'm sleeping for >60 minutes, with those states of deep sleep, I really feel like I'm sleeping. But not when I'm just lay on the couch for 20 minutes.