The student of knowledge should sleep less as long as it does not harm his body and mind. He should not sleep more than eight hours in a day and night, which is a third of one’s time within a twenty four hour period. However, if he is able to sleep less, he should do so.

[Source:  (تذكرة السامع والمتكلم – page 77) & quoted by Shaikh Raslaan (may Allaah preserve him) in Fad’lul Ilm’ page 281]