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Oxford Dictionary says by has the meaning of *‘indicating the period in which something happens: this animal always hunts by night’* Then do we need to say ‘Why does the sky glitter by night?’ and can’t we say ‘Why does the sky glitter at night?’? If we can use both, what’s the difference between the two?

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The expression by night is typically used to contrast someone's nighttime activities to their daytime activities, especially when the nighttime activities are unusual or unexpected. You will see it very frequently in the construction X by day, Y by night. Some examples:

  • ...treated it like a greasy spoon by day and a hotel by night
  • I mean, if it's -- if it's somebody that is -- is living as a man that's a woman or living as a woman that's a man or someone that's a prostitute by night and a -- and a secretary or a lawyer by day.
  • He's a DJ by night, a graphic artist by day. He's multifaceted.
  • ...he dwelled, chief of all worldly American expats, hanging out with bullfighters, jai alai players, and ex-Spanish Loyalist guerrillas, awash in drink and worldly women, fishing by day, partying by night, writing the whole time.

On the other hand, at night is much less constrained as to the interpretation. The preferred thing to say will be that the sky glitters at night, since this is the expected activity for a sky. To say that an animal hunts by night uses the by night expression because we normally think of day time as the time where animals should be most active, based on our own experience.

Another useful note is that by night can almost always be replaced by at night without altering the acceptability of the sentence, but not vice versa.

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Interesting. Makes one wonder why there is no "at day". –  Mr Lister Dec 30 '12 at 14:00
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I would put the difference down to more of a sense of drama with use of by. "The tiger hunts by night" sounds more dramatic than "The tiger hunts at night."

Consider the title of the following film: They Drive by Night, which is a hyped-up way of presenting a movie about truck drivers who are trying to survive in their tough world. Had the film been called "They Drive at Night" it would have sounded pretty ho-hum.

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There's hardly anything "dramatic" about the subject matter in saying someone sleeps by day. Personally I think its an exotic/quaint but declining Victorianism. It's usually at night today, but I don't know why there's no such direct replacement for by day. –  FumbleFingers Dec 30 '12 at 4:37
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@FumbleFingers: The time of day the OP was interested in is night, not day. As for the dramatic, I think you expect that term to mean something more exalted or ponderous than I intend here. Remeber, I said more dramatic; that shouldn't take you all the way to The Women of Try or MacBeth, merely to a '30s movie about truckers. Sheesh. –  Robusto Dec 30 '12 at 5:10
    
What I meant was there's nothing inherently "dramatic" about using by rather than at. It only has those associations because it's an archaic/poetic form. Plus there are doubtless still millions of kids dutifully learning to sing while shepherds washed their socks by night, which all adds to the general sense of "this is a special usage". –  FumbleFingers Dec 30 '12 at 13:06
    
Obviously I meant The Women of Troy. Stupid keyboard. Stupid fingers. –  Robusto Dec 30 '12 at 13:25
    
I'm still trying to work out whether your capitalisation of MacBeth is some subtlety that passes me by (not unlikely, as I fully accept! :). Though I do find it intriguing that you can do things at night, but not at day. And if you do anything by day, it's almost invariably by way of contrast with something different that you do by night, as @jlovegren points out. –  FumbleFingers Dec 30 '12 at 15:41
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