Probably L-theanine? It's found naturally in teas and it's not uncommon in energy drinks/preworkouts. I'm not sure about cold brew though.
There is a minor complication in which if the brain is in a very stimulated state, then theanine can add to the stimulation, but that has happened to me only once in hundreds of times of my using theanine at bed time, and is extremely unlikely to recur now that I know about the complication.
Also, coffee does not contain any theanine.