Why does the old man feel he should risk sleeping in The Old Man and the Sea?

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.