How to knock people out for two minutes or more?

Basically, I know two or three methods of knocking people out. There's the Crusher, (making someone high on oxygen then shoving their chest) the Crouching Thumb (getting high on oxygen then blowing on your thumb until you pass out), and a hefty punch to the temple. Neither are safe, obviously. I've tried all three, and so have my friends- it's just a kind of "messing-around-being-a-douche" thing. It's just fun to do it sometimes, see who can be out the longest. I know you have to be passed out for a minimum of four minutes to begin the risk of taking brain damage. Nobody we've got to do it (including ourselves) has got that far. The best we've had was around twenty seconds. Some of my friends wanna know how you do it for a bit longer, just because they like the "passing-out" feel. I guess. We normally do the Crouching Thumb method. You crouch on the floor, and breathe in and out shallowly and quickly for 30secs, until you start to feel really light headed. Then you stand up as quick as you can, jam your thumb in your mouth and blow as hard as you can until your vision fades and you pass out. Apparently breathing for longer works, we did 100secs on someone once and they were out for about half a minute- but that's about the highest it'll go, thirty seconds however long you breathe for. Is there a way to make you pass out for a bit longer? I mean, when people are injured they can sometimes pass out for an hour. We don't wanna do that, obviously, but maybe just a minute or two. There's no safe way to do it, but you know- safe-ish?