Why do strange things happen when I'm about to fall asleep?

For example, every few nights I'll be close to asleep and I'll feel like I'm falling off of the bed even when I'm right smack dab in the middle of the bed. Sometimes I'll think I'm hearing a voice and wake up, but that's extremely rare. Usually only happens when I'm really tired. Not physically exhausted, just really tired. It snaps me out of my mindset when these things happen and keeps me from falling asleep, which isn't great since I'm already struggling with insomnia. Its 4:44am right frigging now...