Why Do Vampires Appear Only At Night?

...from the Congressional Committee on Non-Human Involvement in the Global Economy:

Current knowledge indicates that vampires fall into a coma-like sleep at the rising of the sun, and awaken only after the sun sets. Despite some reports to the contrary, this appears to be universal. How close to sunrise the comatose state begins and how soon after sunset it ends depends on the individual vampire. Direct sunlight destroys any vampire. It is unknown whether vampires seek cover because of their involuntary comatose state during the sunlight hours, and choose entombment or burial for the purpose of seclusion and protection against attack, or if burial is necessary for other reasons, such as rejuvenation. It is not known if coffins lined with earth, contact with earth, or protection by earth are necessary. Vampire sources of information on this matter are unreliable at best.

Note that the following photo is obviously not of a vampire. There are several cues: one that the picture was taken in full daylight, two that the supposed "fangs" appear to be candy corn, and three that it is a photo (it is impossible to capture an image of a vampire, whether in a mirror, on camera, or with any form of media--except audio media, interestingly).