Even if you realize with disappointment the morning after that the book under the pillow did not make you smarter, the advice points in the right direction, because while you sleep, your brain is active.
Sleeping makes you smart, because at night what you have learned is transferred to your long-term memory. What was learned during the day is consolidated at night.
In addition, maintenance work takes place at night, i.e. toxic substances are flushed out so that new information can be absorbed more quickly and effectively in the waking state. The molecular junk and synaptic garbage that have accumulated in the cells are removed or put to new reuse.
Sleep is a kind of nature’s miracle pill for the health of the mind and body.