I knew a guy who slept that little and he thought he was fine with it but he routinely fell asleep for a few seconds at a time during various times/activities and, strangely enough, had no idea he had fell asleep. I had to record him doing it before he believed me.

Note that this does vary from person to person. I know people who can sleep anywhere but I personally find it very difficult to sleep sitting up. I pretty much never sleep on planes, for example, no matter how little sleep I've had or how long the flight is. So in this commuting situation I would try to sleep, but probably only be asleep for half the time and never get to the REM stage.