I can get pretty sleepy during meditations too. Sometimes it's not so bad, but when I'm really tired and really trying to meditate, it's super frustrating. BUT, check out the graphs
in this link. There is one for "sleep need" and I find that it correlates really closely to me.
Yesterday, for instance, I had a long sit after work. At first my mind was fairly clear (M&B, but I quickly become a bit drowsy and somewhat restless (3 Cs). Then I became super alert and had some energetic rushes and pressure in my forehead and fast vibrations (A&P). Then I got drowsy again and really foggy and meditation was pretty difficult (DN). Then I got alert again, but not engergetic as the fog lifted from my brain and things were crystal clear (EQ). This whole progress was over 70-80 minutes or so.
So when you get tired, maybe try upping your effort and seeing what's on the other side, especially since you say that you just had exciting A&P stuff.
-T