Whether it is down or synthetic, you need to air a bag in the morning by hanging it up or spreading it out. If you neglect to do this, your bag will feel clammy the next time you get into it.
I always aim to do it, but much of the time it simply isn’t possible. It’s either raining or thick mist, and even when the sun is out there’s often nowhere dry where you can hang or lay it – in the morning fences have drops of condensation and damp posts, tree branches often have wet leaves, walls and rocks have damp moss on them, while the flysheet of your tent is usually wet with condensation.
If you don’t get the chance in the morning, consider dragging it out and hanging it on something if you can when you stop for a lunch break. (You could dry your tent, too, if you have time and there’s somewhere to like a fence or a five-bar gate to hang it!)
I should perhaps caution that even when you are able to hang it up, the lightest of breezes is capable of making it fall down onto wet grass or ground, so you’re constantly having to race across to pick it up.