Yes I have this sometimes. It's even accompanied with a sort of tingling in/on my head that starts from the back/crown of my head. I always realize it's happening when I start to lose track of what I was thinking about, and can't recall thoughts I had just before I realized that I was falling asleep.

By this time it's too late to just let it happen, and I'll have wasted a 'chance' to fall asleep so to speak.