Naptime?

What is with people sleeping on the train. Yes, this includes me. Why is the train the perfect place to nap? Is it because the constant vibrations? Is it the sensation of moving while not being active? Or is it just because its 4am, and we're all sleeping off last nights alcohol? I think its some combination of all 3. but there are two types of sleepers. Those who fall asleep with pleasure, and those who are gripping onto staying awake. you can find the second type by paying attention to how they act during stops. Do they pop up for several seconds before lulling themselves back into their dreamland? Yeah...... that's us. Its also easier to get away with not paying attention to people acting like fools when your asleep......just make sure your wallet/purse/backpack are in safe places. Seriously? Seriously!