Hmmm an interesting question. It always depends for me, I find sometimes that by 9:30-10:00pm at night I've had enough after school all day and studying for a large portion of the evening. It quite often tires me out. On the other hand, finding that motivation and the willingness to pull yourself out of bed at 6am in the morning takes a strength I often do not possess. Maybe I shall experiment more and find out what works. Also, the effectiveness of each option is up to the individual that is studying. Maybe you should try both, see which one works best for you