Hi Jun, This is a good question. Essentially, 'by' and 'at' refer to different types of prepositions. But, to make things a little less complicated, think of saying 'at night' as meaning doing something at a particular time in the night. I will go to sleep 'at night', for example. So, maybe 9 pm. Alternately, 'by night' suggests that something needs to be done during the night - so, not necessarily at a specific time, but definitely in the night time. So, for example, 'I will do my homework by night' , meaning I will not do it during the day, but rather by night - in the night - so to speak. I hope this helped. Nice question. Take care.

Yes, I agree with Ashley this is an interesting question, that I have been asked before as well. The way I explain it is that they both mean practically the same thing in that "by night" can almost always be replaced with "at night" and your sentence will still make sense. Having said that however, "at night" cannot always be replaced with "by night" and still make sense. Also the term "by night" is often used to lend a sense of intrigue or drama to what the speaker is trying to say:- " Lions hunt by night" sounds more interesting than saying "Lions hunt at night"