Are they comfortable enough to sleep on? is it allowed? is it wise?
It's possible, but they're not especially comfortable and don't recline. Make sure to book in advance to reserve a seat.
Dedicated sleeper trains are considerably more expensive, so if you're flexible on time it's much cheaper to just stay overnight in a cheap/pod hotel. almost 10 years ago
Definitely possible if you reserve the green car seats in advance. The dedicated sleeper trains have all but been phased out these days (there are still some though). Japan is also extremely safe so you shouldn't worry about sleeping on the train as no one will disturb you. Most people sleep on the trains here so yes it is allowed. almost 10 years ago