Vitamin D! I've been deficient. Turns out: If you live in San Francisco, most of the year it's impossible to get enough from the sun because of our high latitude. To remedy this I've been taking some insane 50,000 IU supplement. So far the only side-effect has been sleeping more deeply and having more dreams, which is a good thing? Anyways, that's a pro-tip I wish someone told me years ago.