It's radiant heat loss. Always worse from a roof surface because it's pointing at the sky, so the roof radiates heat (especially to a clear night sky) and the temperature falls below that of the surroundings (walls, floors, surrounding ground etc). This is then a magnet for water vapour which condenses on the cold surface. Insulation and a vapour barrier on the inside will definitely help, even if you don't heat the shed. Ventilation can help, but it won't stop the radiant heat loss at all, so this will happen even in an open sided shed (and it does so in my log store).
The walls (and floor maybe?) would benefit from the insulation and vb treatment as well, along with a coat of something waterproof on the outside - single skin shiplap will be almost saturated at this time of year and that is where a lot of the water vapour will be coming from inside the shed, so you need to stop the water getting in through the wood.