Night takes the preposition 'at'. You always say, for example: I always run at night. You never say 'in night'.

Yeah, "in night" is never used. However, "in the night" is used in some cases, but always use "at night" to be safe. Just know that "in THE night" is possible.

The preposition "at" is the most common. You can use "in the night" with the same meaning as "during the night". These "in / during" are usually used to talk about the night that has just passed. I woke up twice in the night = I woke up twice during the night When you are talking about one particular night, you use "on". On the night of Monday 15 October. When you are talking about something that you usually do in the night-time, you use "by". Some animals sleep by night and hunt by day.