Bed bugs are among those kinds of insects that sleep or hide during daytime to finally come out and hunt for food at night time. Call its mart, but such insects have amazing biological clocks that know when it is time to go out and look out for prospective and unsuspicious victims.
Bed bugs do not eat anything but blood. However, the blood needed by bed bugs for nourishment should come from mammals, or hot-blooded animals. Again, they might be smart enough because they can tell by mere smelling if a prospective victim or host is hot-blooded or not.
Bed bugs suck blood of people. But their food requirement is not limited to people's blood. They can also suck, as alternatives, blood of dogs, cats, birds, or whatever animal or pet they can get their mouth into.