However, there is no hard and fast rule for this. After spending eight hours in school/college and having 7-8 hours of sleep, whatever time is left for self-study, you have to decide how you are going to use it. There is no scientific evidence who can evidently say about the right timing. Some say, night time is best for creativity and that is the reason, all artists, writers work in the night. Some say, in the morning, you get sunlight, have fresh air, which gives you a nice environment to focus. You can divide the time, according to suitability and when you do feel that you are able to grasp the most. Your focus does matter and not daytime or night time. Do not get stuck into this debate. Everyone has their own pace so what suit best, Prepare accordingly.