Tj, the more light during the lighting period, the better. Having your closet door open is ok UNLESS it leads to lost lumens. Ideally, you want to reflect as much light as possible back onto the plants, so having an open door means you will be bleeding some light that could be hitting the plants instead. During the dark period, you want it as dark as possible.
Jimbo, you raise an interesting point of nature vs. artificial growing conditions. In nature, the plants have moonlight part each month. I am no scientist and have no idea how many lumens a plant would receive during a full moon. I would venture not a significant amount, especially in the spectrum that's important for a plant to grow. Ideally, you want no light during the dark phase, including street light, night lights, etc.
