In the night/at night

at night

a night OR every night

Neither "at night" nor "in the night" sounds good to me.

Ah, Ms. Jandi-- onto the bandwagon, are we?

1. They ran away in the night. I like this one best; it sounds the most surreptitious.

2. The enemy attacked by night / at night / in the night. All three sound fine here, with slightly different nuances-- see below.

3. All week, the enemy attacked our left flank by night / in the night. 'At night' here sounds too mundane for the situation.

'At night' is the matter-of-fact, objective form. 'By night' suggests the timing of an activity. 'In the night' suggests clandestine matters. That is how the three forms seem to me (in addition to idiomatic concerns, like 'I always watch TV until 11 o'clock at night').

We usually use "at night" to express an action that occurs at a more or less
precise point in that time period.

"I go to bed at night."

We use "in/during the night" to express an action that occurs over a period of
time within that time period.

I sleep (usually for eight hours) during the night.

The stars appear at night, but they shine "in/during" the night.

I hope that this helps.

Peace,

David