I also get sleepy at gatherings. It's something really subtle and hard to defend against, but brutally simple.

It's the food and air quality. There's a lot of people in an enclosed environment eating too many carbs (or too much in general).

Timing is critical and I'm on a mission. Before the heavy food comes out, I grab a seltzer and a small salad. I convince some people to join me on the patio for a drink. It's important to arrive late enough that people are eager to break away, but well before any food ceremony.