It is how long the sun stays up. It is winter, therefore the sun is out for a shorter amount of time, hence SHORTER DAYLIGHT HOURS. That means that her coat will grow FASTER because of the SHORTER days.
Now that we are past the winter solstice the day light hours will INCREASE and soon your horse will start to shed out her winter coat. It's not going to happen tomorrow. But in a month or 2 it will be noticeable.