Having a cabin with a balcony on a cruise ship is nice for breakfast, reading a book or relaxing and even taking a nap.
I would not recommend sleeping all night on the balcony though unless you really want too.
I've seen a few older people on the cruise ship who did drift off to sleep and slept the whole night but you might get smoke on you during the night from the smoke stack.
I would recommend just taking a nap out on the balcony of the cruise ship but then going to your cabin and sleeping in your comfortable bed at night.
The moving motion of the cruise ship can help some people sleep while others may have trouble sleeping on the cruise ship.
So you could take a nap or sleep on the balcony of the cruise ship but I would not recommend sleeping the whole night on the cruise ship balcony.