In the previous sentence, there was no "at" after "stay up", but here it is "stay up at". Is there any difference between two(stay up vs stay up at)?
The key is actually in the word that follows: night, versus tonight. To indicate time, "night" is usually preceded by "at" (at night), whereas "tonight" can stand on its own. The form of these two are different, but the intended meaning is similar. Keep in mind that at night can be a general expression, whereas tonight is more specific. Ex: I stay up at night to do homework. Ex: I will stay up tonight to do homework.
Get more Native English Expressions >