It is clear from the text that the old man is exhausted, and not only that, but there is going to be bad weather soon, so he needs to make the most of the opportunity that he has to sleep now before a storm hits and he might not be able to. In addition, the fish is "calm and steady," and so is unlikely to try and fight the ties that bind him. Note the way that the old man sets up a system so he can still have very firm control over the fish, even whilst in sleep:
My right hand can hold it as long as it is braced, he thought. If it relaxes in sleep my left hand will wake me as the line goes out. It is hard on the right hand. But he isused to punishment. Even if I sleep twenty minutes or a half hour it is good.
Thus the old man recognises the importance of sleep to strengthen him for the struggle between himself and the fish that he sees ahead, and he is able to set up a system to enable him to maintain control of the fish and not lose him while he sleeps.
we must be aware that old people do not sleep long even at night , in fact they consider sleeping as a thing that wastes time in their short life .
Hemingway himself suffered great pains from insomnia. He understood the importance of sleep to one's efficiency.