Have a look at the science behind DARPA research investigating the possibility of enhancing memory using tACS. Dr. Vince Walsh is the lead investigator. and team from NPR reporting.
In a multi-year study at the University of New Mexico, volunteers received a fraction of 9-volt battery’s worth of electrical stimulation to their scalps while they slept at the lab. When they woke up, they were asked to play a video game they had learned the day before. Turns out that subjects were significantly better at it after the night spent in the lab.