Well the problem with that is natural sun light changes every 15 min. Every 15 min the color, temperature, direction, and brightness of sunlight changes so trying use that at reference is going to be nearly impossible. Capture a photo, or a bunch, with a nice camera so you can really study the light. As for the technical…
Are you using final gather? This will help calculate the light as it bounces around the room and brighten it up a bit, providng a more realistic lighting situation. You can create a photon map specifically for this in the Indirect Illumination tab under the Render Setup menu. I would highly suggest doing some research on…