I would have to agree its related to your astral body preparing for sleep.
I have had the sensation of spinning whilst falling asleep multiple times although it occurs randomly. I find it to be quite a pleasant experience and
have learnt how to manipulate it so I can speed up or slow down the spinning sensation. I feel it as my astral body and have some experience with
astral projection which is why I believe it's my astral body spinning and not mental hallucination or delusion.
I have yet to learn how to turn this sensation into a direct astral projection experience, although there have been times when I have fallen asleep
afterwards and entered AP.