I have the same problem when I use sunlight. Artificial lighting is much better as it gives you much more control. I use a 5600K led lighting unit or a ring light depending on what works best( depends on the coin). Its my understanding that if you are going to rely on sunlight early morning light is the best.
Some people recommend cheap Ikea LED lighting. I tried that at it was useful to a point. However my Camera, a Sony, has a fixed(Doesn't move) mirror. That mirror only allows 70% of the light passing through the lens to get to the sensor making the Ikea lights just a little too dim, hence my going to a much brighter LED light source.