One should sleep whenever he/she's tired.
Anyone who has the luxury to choose what to do with his/her time is extremely successful or at least happy.
And also the quality of sleeping is more important than the quantity. No point sleeping 2 more hours if you feel rested and rejuvenated after only 4-5 hours. And again, lucky is the one who can get to the "office" at 4 AM or 1PM whenever he/she is awake and feel like working...