How much ventilation do you have?
Infectious Bronchitis is a respiratory illness where the symptoms seem to be more noticeable/worse at night, so that's a possibility.
It's really hard to know unless you have some testing done. So many things can cause respiratory symptoms. Symptoms noticed at night and in the morning if they are in a coop, could be environment or respiratory illness. Have you changed the bedding recently (dust) or closed more windows/vents than you normally do?
Mine sleep inside next to my bed lol so they go in and out of the kitchen when they want. Which they do only when they want a treat. So I have no idea what happened. I am on stealth mode any noise I hear I am thinking my silkie sneezed lol.
So if I understand don't give anything unless I start seeing signs?
I have been putting that Rx stuff that is like Vicks on the towel just under her and under her wings and it seems to help at night. I had taken frizzle into the vet one time Just for a chk up and I asked him about it what it was and he told me it does nothing because chickens don't have some type of membrane in the nose like we do. But I beg to differ it sure helped and glade i bought it for no apparent reason at the time lol.