The verse says “And Hashem made slumber fall on him!”, and Chazal say that there is an allusion here to the fact that the beginning of a fall is sleeping as because he sleeps he does not occupy himself with Torah or work. This is especially true for a learned man as the “Rinah” (Song) of Torah is only at night and this is a special effective time to mourn the destruction of our holy temple by the saying of Tikkun Chatzos and to pour our one’s heart and ask for forgiveness and one can reach much higher levels of spiritual perfection at night than during the day. In the same way that it is good not to sleep during the day without a special reason so too it is good to sleep in the day in order to stay awake at night.
A person must make his own calculations as even though they said that it is good to be awake all night on certain nights (such as Shavuos and Hoshanah Rabbah), nevertheless if he sees at the end of the night that sleep is starting to overtake him, and he is worried that he will be unable to pray in the right way, he should lie down to rest a little close to dawn in order be able to arise and concentrate fully on the prayers. Sleep such as this is considered a great deed for him. About such things it says “How good is a man who sustains his matters with justice!”.