I rode yesterday and it was -30 with windchill, I'm not sure what the temp was without windchill. BUT.... I rode in the back where the wind is mostly blocked and it wasn't bad. When it's this cold I don't ride hard, generally just walking maybe a short canter/trot. I work on leg yeilds, hindquarter/forehand turns, side passing, lateral flexion, walking serpentines, figure eights, etc. Basically continue suppling my horses and increasing their responsiveness without making them expend a lot of physical effort. I would think you'd be fine for some light exercise in an indoor.