i find there are several answers to that. here are my fav 2 ways:
1)only the worlds elite need to worry about something like that. i think it was the russians, surprise!, that showed 2 times of the day that test levels are highest, 10-12 and 3-5. that is if one sleeps regularly from 11pm to 7am. i could be wrong on the exact numbers but that is the gist and hopefully you get this point
2)which ever you will do consistently. as that will give you better results then the time of day.