I like to water flowers or fish when its night time.
I also check out if anyone of my villagers is at the cafe and talk to the ones which are still awake. 4am music is my favorite, thats why I update my dreamtown always at that time.
Ikr? My favorite music is at 5am. so calm and relaxing!
At night i'm most likely catching beetles, sending letters to villagers, watering plants, or just talking to the villagers who are night owls.