SIN is supposed to be a 24 hour airport but most of the shops and food outlets do close late at night. In fact, after about 11 pm, you can usually find an open newsstand/bookstore and a small duty free outlet, perhaps one or two Asian fast food type restaurants, and IME almost nothing else except the lounges, transit hotels, and free movie theater. I suspect there must be a 24/7 convenience store (combination drugstore/pharmacy and small grocery store) but I haven't found it.