I could say
that I'd be up late studying
or
I could say
that I couldn't sleep tonight
(just tonight, random sleeplessness)
or
I could say
that I got distracted
(by Wikipedia, the CDC, Edmodo)
or
I could say
that I fell asleep with the light on
(at my desk, with my book, and my laptop)
or
I could tell the truth
(that I don't sleep, that I hate sleeping, that if I sleep more than four hours it's as bad as pulling an all-nighter)
or
I could stay up by cellphone light
(so no one can see that I'm up)