Putting a sheet on your horse in the winter can KILL your horse! The horses winter coat works when it is fluffy. A sheet alone will actually slick the coat down and take away a lot of the warming qualities of it! Sometimes NO blanket is BEST!
Its better to be over blanketed than under blanketed during winter. If you have too light of a fill blanket it will slick down the coat and not be warm enough for your horse.
When in doubht. If your horse is plenty wooly. DO NOT BLANKET! I have a few mares who do not grow a good winter coat that we blanket. THe others I leave bare!
I live in florida and blanket my "senior" mare at night if it will be below 60 degrees! And she LOVES her blankie!!! She shuffles on over to her stall as quick as she can when she sees it coming, "talking" the whole way..... on the rare occasions that it drops below 40, I even wrap her legs with some "homemade" wraps that I made for her out of old bath towels.....
I would just like to add, I have always been taught to do up the rug from the back leg straps first, then the chest straps, then any other straps last.
This is because if you do up the front straps first, and your horse spooks, or something happens and your horse ends up loose or panicking with the chest straps done up only, that blanket is going to end up flailing around in front of your horse until it either rips free, or your horse trips and falls on it.
Whereas if you do up the back leg straps first, and your horse spooks/gets loose/whatever, and he runs, the blanket is going to flail behind him and actually come off the back legs after a few strides without injuring your horse... Yes, I have had this happen -_-