"I believe that the brain during sleep is not at all totally detached from the external world," says the lead researcher, "and stimuli, including those stemming from the environment, are probably incorporated into dream content more often than people observe or are aware of."
There has to be a lot of truth to this. Having two dogs that are not forbidden from getting up on the bed, I keep the bed covered with an extra "dog coverlet" to keep sand and dog hair from getting into the blankets and sheets underneath. Oftentimes in the summer though, I just wind up crashing on top of the bed, not pulling off the coverlet.
I've noticed a few times, when the coverlet is getting time to be cleaned and has a bit of sand and dog hair in it, that I've had dreams involving dishevelment and filth. Like living in a condemned house or being in a dirty hospital, etc. Weird.