At night or In the night
At (= when it is/was) night (= night-time)
*"At the night" is not idiomatic.
In (= during) {the night} (= (i) the current night; tonight (ii) the night that has just past, or (iii) night-time.)
(i) "After you go to bed, you may hear a noise in the night"
(ii) At breakfast he said "Did anyone hear noises in the night?"
(iii) The noises are caused by wolves - they come out in the night."