The forced distribution of hot air through the duct creates a fairly uniform temperature in the cell, but consider that in the attic it will be a little hotter than below, take this into account so as not to lower the stove too much if someone is sleeping downstairs.
Personally I prefer cool to warm, so I leave the attic porthole slightly open (a couple of fingers, no more), in order to reduce heat stagnation and, as already mentioned, avoid condensation.
Even the children, when they were small, slept on it, naturally with a protective net, positioned lengthwise (head on the side of the ladder and feet on the side of the low ceiling). By the way: during the march, everyone is always seated , with the belts or the seat, it is forbidden to stay on the beds .
If the air ducting does not reach the attic, during the night put a blanket (preferably padded) along the lower part as a draft excluder (pack it without pressing it), it will protect those sleeping on that side from the cold of the wall. The air distribution often arrives around the beds, this means that there is a warm veil on the wall which avoids condensation and cold. But that's not always the case, and if it's really cold and you feel the wall frozen, do the same as for the lower part of the attic, try to keep the mattress detached from the wall by placing a blanket rolled up lengthwise, to form a nice large sausage, like a draft excluder. This expedient can be especially useful in those beds which have the side wall of a locker door.