Why do vampires only come out at night?

I have an I-search paper due in my English class and I decided to do it on vampires. I need to know...Why do vampires only come out at night? Please be specific and give me a few reasons why. ( and please nothing about Twilight like "Because they sparkle in sunlight.")
Update: I know, I'm pretty sure vampires AREN'T real, but I'm doing my report on them anyways. I'm just a kid, let me believe what I want.
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