Actually, yes, I recently read that what you are saying is true. Wish could recall the source. You don't even have to be asleep. You can practice a skill in your waking mind. And it can help you attain things like better physical strength, co-ordination. Maybe because the more you do something, the more neurotransmitters develop in that part of the brain. You also, supposedly, develop more blood vessels in whatever part of the brain is involved with whatever activity you are involved in.

While you have as many brain cells as you will ever have when you are born, your brain does, apparently, kind of grow or evolve. If you do a lot of math, you become more adept at math, not just because you practice, but in a way, you are literally exercising that part of the brain: developing more neuropathways there.

I wish I could remember exacty what I read. Say you are interested in weight training, if you vsualize yourself training with the weights, supposedly, people who do that are actually better at building muscle and developing that muscle at a faster rate. Cool.