I had this experience at Big Bend State park that makes me think they are. I didn't bring enough water and camped in the primitive area. At night, I was dehydrated pretty bad. When I finally got a little sleep (it was tough to say the least), I had this vivid dream where I put a pebble in my mouth and started sucking on it to make saliva. Then I woke up for real, and I knew it because there was a lot of wind IRL, that wasn't in the dream. So I took out a coin from my back, put it in my mouth to make saliva, and got a little bit of relief. Enough for a couple hours until it was dawn, and had enough light to hike down to the restroom area.

I don't know where I got this trick. Likely some survival show or some novel. But I don't have any background in survival, otherwise, I would have brought a lot more water.

So my brain knew there was a memory that could help and made up a dream about it is my theory.