The military has been doing tDCS (direct current) experiments for a while for increased learning & performance.
I had a friend who used to swear by tDCS and did it very frequently. He attributed all kinds of mood & cognition improvements to it, including substantially reduced need for sleep, improved eyesight (he was born partially blind), etc. He also complained about migraines a lot and liked to experiment with all kinds of supplements, so YMMV.
Tried tDCS myself a handful of times, not for improved learning but for improved focus. If applied correctly it definitely has an effect that I'd consider better / stronger than a cup of coffee. I wouldn't feel comfortable doing it regularly though.
There's a subreddit that used to be pretty active: https://www.reddit.com/r/tDCS/
And here's a good Radiolab podcast, including DARPA Sniper experiments, etc: https://www.wnycstudios.org/story/9-volt-nirvana/