[purerandomness]

Alternative hypothsis: People who stay up late have a higher probability of going out, enjoy more occasional drinks, are more probable to smoking or take recreational drugs and have more sexual partners and generally enjoy more riskier lifestyle choices, all of which contributes to the statistics showing a shorter life expectation.

[barrkel]

And of course all of this sounds substantially more fun.

[S_A_P]

Which may also lead to a greater since of community and reduced loneliness which contributes to longer life.

[cowsandmilk]

Because no one ever found a sense of community from their morning bike ride or running group or being monogamous and having a family... The idea that sexual promiscuity and doing drugs and drinking to lower social inhibition are what creates a sense of community is pretty misguided.

[S_A_P]

Not what Im advocating. just providing a counterpoint to the argument that the only outcome to staying up late is that you are making choices that shorten life. Staying out late certainly can contribute to the OPs theory, but that isn't the only outcome. Reduction of stress, finding a community/partner/etc is definitely something that hanging out late can provide. the time of day you feel most awake doesn't mean your default is self destruction.