This feels completely stupid, but I noticed what seems like an unmistakable improvement in my sleep quality (more restful, less waking up too early) a few days after I adjusted my diet to include a lot more fiber intake. After googling it, it seems like it might be supported by studies, but I'm always a bit skeptical. Either way, if you have trouble getting good sleep, it's another dietary consideration to make. Increased fiber intake seems like it mostly has upsides.

(The other advice in the article is obviously also solid, but none of it, including changing caffeine habits, really ever seemed to do much for me. So I guess it will vary a lot from person to person.)