Let's cut to the chase. Say you decide to go on a fast, and so you effectively starve yourself for a week. At the end of seven days, how would you be feeling? You'd probably be hungry, perhaps a little weak, and almost certainly somewhat thinner. But basically you'd be fine. Now let's say you deprive yourself of sleep for a week. Not so good. After several days, you'd be almost completely unable to function. That's why Amnesty International lists sleep deprivation as a form of torture.