Its been very interesting to monitor the horse's behaviour, especially after dark. Normally we have so many wet, overcast nights that its no surprise that they show a preference for being in the barn - dry, sheltered, lots of haylage!
Recently though its been dry and clear for many nights, and what seems to happen is that the horses prefer the barn if its dark, but prefer being out on the track if there is a decent amount of moonlight.
Obviously in a normal winter its so cloudy up here that the moonlight isn't that bright, or else when there is a moon its pouring with rain...(!) so its not something I've noticed before.
Bob Bowker described when he spoke at our conference last year how movement patterns in horses are greater during the day than at night, so this would tie in with that as well - I wonder if anyone in a more salubrious climate than ours has ever monitored whether horses cover more miles when there is more moonlight? Or maybe thats just moonshine :-)