Electric Power Rationing???

Brown outs usually do not happen in the middle of the night. Also they would not happen night after night without public notice. You are probably being affected by a number of factors. It could be a neighbor. Big industrial customers or mines sometimes have huge power demands. Some of these customers can affect the power for over a hundred miles. If you live in a high rise apartment building you might look to see if they are doing something automatically. Since the change is exactly 8 hours in duration it is unlikely to be a malfunction. Because of the exact timing I would say it is a large industrial customer doing some automated process. You can call the utility and ask them to give you a 24hr voltage reading. This is not easy but it can be done. They then can start looking for the cause. The sad thing is that there may be little they can do for you. Sorry, but that is just the physics of electricity.