L-Theanine? goes great with coffee (not mixed, just taken near the same time) - smooths out the jitters. Probably why Tea is more 'calming'

Could you point to any research? I asked many doctors but nobody really knew about the substance nor could tell me whether it's safe to supplement.

FWIW, 200mg of L-Theanine is included in my workout recovery supplement. Research seems sparse but here's what they say about it: (scroll down, click ingredients tab) https://athleanx.com/athleanrx/reconstruxion

examine.com is often a good starting place and lists many studies.

https://examine.com/supplements/theanine/