Why do I feel sleepy after a crying?

Ok, so I do understand why I would feel sleepy after a good cry if I was tense or upset, but I just gently cried after seeing a family re-unitited after 15 years. It was a sweet story and I wasn’t expecting to cry but it was very touching. I’m not the emotional type but I found myself wiping away tears of joy after watching the program. Then I found myself yawning every few seconds. I feel so sleepy now. So forgive me if I don’t reply tonight. I don’t think I can stay awake any longer. Just a heads up. I haven’t felt stress all day. If anything unusually happy.
I thought that this would only happen with sad tears. Not happy tears.
So what gives?