Bri, yea. They would need to be under lights before all the light of the day is gone.
Also, if you want to save electricity, you can set the lamp to be off during the day when it's light. Normally, when people say they are lighted for 13 hours, it means 13 hours of light per day that their horses are exposed to. Not necessarily lamp light. Just pay close attention to the timer when you set it. Those things can be kind of weird. Remember to reset for daylight savings.
And get clear heat bulbs. The red ones don't do much as far as lighting.