People 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. People 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. Also, ---- in the evening ---- in the afternoon ---- in the in the morning / refer to a limited time but ----- at night ---- at noon refers to a specific point in time. For example at night= 12.00 a.m at noon= 12.00 p.m at dawn= 6:45 a.m at dusk= 18:45

These are not distinct times of the day, but "in the evening" would refer to a time before "at night". If you want to know which expression to use in a situation, in general, I would use "in the evening" for times between 5:00 pm (17:00) and 9:00 pm (21:00), and I would use "at night" for times after 9:00 pm. I think that would be a useful guide. I hope this helps.