Is sleeping while not lying down (for example in a train or in a car) not as good as sleeping lying down in bed? How do you compare 1 hour sleep in a car vs an additional 1 hour night sleep in bed?
There is one major problem with sleeping in a sitting position - gravity. When you lie down both your blood pressure and heart rate decrease. This allows your body to relax more and, perhaps, increase the rate at which you recover from illness or injury. I'd imagine if you're body didn't have to work as hard getting blood to your brain while you're asleep, you might sleep better.
Everything that goes up must come down and your head is propped awfully high on your shoulders. Maybe I'm a light sleeper, but having my head bob around makes it hard to sleep, not to mention limbs flopping around when muscle relaxing REM kicks in or just loosing your balance completely and falling out of your chair. I think I'll stick to lying down.