Search terms used: various combinations of Japanese, folk remedies, home remedies, cure, traditional, sleep, insomnia. I also read a 10-page article on JSTOR called "Some Japanese Beliefs and Home Remedies."
My character complains that he couldn't get to sleep last night, and one of his friends suggests a very common, simple remedy to him. I want it to be something as mainstream as hot milk or chamomile tea being used as a sleep aid in the US. Preferably not something that sounds incredibly superstitious or old-fashioned, or would take too long to describe (like, I'm not going to say something like "Did you try making an infusion of goat's milk, honeysuckle, and cat's tears, poured into a crystal bowl under the light of a full moon for one night, then boiled in a calfskin bag? It totally works!") The less exotic the remedy sounds, the better, and it doesn't have to be uniquely Japanese, as long as it's something that would be obvious for an average Japanese person to try. The character who makes the suggestion is originally from a rural area but has been living in Tokyo for many years, if this is relevant.
I've tried looking for Japanese home remedies, but all I could find were cures for the common cold and things like that.
EDIT: I think I'm going to stick with the warm bath suggestion, unless anyone has any better ideas.