Basically when you wake up your brain releases the adenosine that it produced during the night that controls how tired you feel. And it takes some time for it to clear about 60-90 minutes, where as if you drink tea right when you wake up it can block the adenosine from clearing leaving you more tired. Idk if I explained it right but Im pretty sure thats the gist of it

Edit: a few words for clarity

If you are drinking it first thing in the morning wait 90 minutes after you’ve woken up.