I would have to assume that if you're sleeping on the subway, then it means you are extremely tired. It's not like it's the most comfortable place in the world. We've seen how other people sleep in some of these pictures. You tell me if any one of those looked comfortable. I'm pretty sure none of them looked ideal. This lady is in the classic sleeping in public pose, which is the same position that gives you permanent neck injuries. It's better to be well rested than to have a functioning neck.
I wish there was a way for humans to close their mouths when they sleep in public. No one needs a vent of stinky breath in the middle of the subway train. Plus, everyone knows that your breath gets worse when you're asleep. I don't know why, but it just does.