What is the difference?

[bartholomew]

What is the difference between at night and in the night?

[jno]

Not much, I don't think. 'At night' can be used to mean every night ('he goes to work at night'); 'in the night' may refer to one particular night. But I don't think either of those is an invariable rule.

[charliep88]

here is my 2 farthings worth  at night may refer to things that occur  at night time. in the night may mean in the darkness of the night out of doors perhaps?

[flaming]

I think it's a question of modern against 'olde worlde'.  'In the night' is a bit more poetic, romantic like 'in the night, he came to me' whereas 'at night, he came to me' somehow sounds more prosaic.  Yes I would take 'at night' for use describing things like 'I work at night' or 'there's a good programme on at night' or  'I go to bed at night'.  In none of those examples would I use 'in the night'.  So it is more a question of concept and context which term is chosen as having the right feel, even though they basically have the same meaning.  

 Night night!

[QmunkE]

"a theif in the night"

"a theif at night"

"in the night" could be said as referring to the "night" as a "thing", rather than placing a time on something.