Here’s an interesting quote from Prentice Mulford:

“The spirit is weak at night, because its forces have in thought been sent in so many different directions during the day that it cannot call them together. Every thought is one of these forces, and a part of your spirit. Every thought, spoken or unspoken, is a thing, a substance, as real, though invisible, as water or metal.

Every thought, though unspoken, is something which goes to that person, thing, or locality on which it is placed. Your spirit, then, has during the day been so sent in a thousand, perhaps ten thousand, different directions.

When you think, you work.

Every thought represents an outlay of force. So sending out force for sixteen or eighteen hours, there is not at night sufficient left in or near the body to use it. The body therefore falls into the condition of insensibility we call sleep.

During this condition the spirit collects its scattered forces, its thoughts which have been sent far and wide; it returns with its powers so concentrated to the body, and again possesses it with its full strength. It is when scattered as so many scattered rills of water trickling in many directions. Put all these together in a single volume, and you have the power that turns the mill‑wheel.”