Horse's don't need to lay down to sleep except to get the 30-60 minutes of REM sleep they require daily. Laying down is also hard on the internal organs, that's why they are equipped with a "stay" apparatus.
We also try to let our horse's "hair up" before we start putting blankets on, but I've been in barns that have blankets on as soon as the temps go below 60F.
Being a boarder you sometimes are at the mercy of the BO. You know your horse best, he'll let you know if he's happy or not.