Why did I woke up even if I didn't sleep for 23 hours?

So Im planning to stay awake for 36 hours, and just 12 hours and 30 minutes remaining, I watched a 2 minutes video and I think I suddenly fell asleep, my iPad is in 5 minutes auto lock so I dont think I slept for more than 10 minutes, when I woke up, the video was 25 minutes ago, my question is why did I woke up? I mean in my situation, when I fall asleep, I'll never wake up until I sleep for more than 6 hours, is it because I'm sitting while leaning on a wall on my bed? Or is it because I drank 2 cup of coffee maybe 1-2 hours ago, what's the reason? And when I woke up, my eyes are not sleepy anymore, why's that?

Read more: Why did I woke up even if I didn't sleep for 23 hours?