I agree that it sounds like meditation. In a yoga class I attend, which combines an hour of light flow yoga with about a half-hour meditation (first about 20 minutes prone on the back, and then 10 minutes sitting cross-legged), I often enter a sort of immediate REM-like state. While the teacher was talking about clearing our minds, I was having wild, vivid, waking dreams. It's pretty fun to get into that state, and it definitely makes me feel good afterwards. The OP's "napping" sounds a lot like this.