What is Santiago's alarm clock in The Old Man and the Sea?

The answer to this question can be found in the first
evening that is described in this book, the night before Santiago embarks on his single
voyage to try and catch a fish. Consider the following conversation between Santiago and
Manolin:

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"Good night then. I will wake you in the
morning."

"You're my alarm clock," the boy
said.

"Age is my alarm clock," the old man said. "Why do
old men wake so early? Is it to have one longer
day?"

Clearly, Manolin refers
to the way that Santiago wakes without fail very early in the morning. When he calls
Santiago, half-jokingly, his alarm clock, Santiago responds by saying that it is age
that is his alarm clock, and then reflects on the way that as humans grow older they
seem to need less sleep and are able to wake up automatically earlier and earlier. The
rhetorical question at the end of this quote is actually quite poignant, as it focuses
on the approaching inevitable death of old people and the desire to try and make the
most of our lives while we have them.