I created a landscape using the Landscape Mode, then started to create a day/night cycle, but when the sun is bellow the landscape (its night), i get a strange soft light from bellow the landscape in every single actor.
Thanks 
You have a few options:
- Turn the light off when it gets below the horizon
- Put a large lightblocking mesh under the landscape
- Use a two-sided landscape material
The first one is your best option, as it improves performance since your light won’t be rendering while underground, but you’ll probably still need to place a light blocking mesh under the world unless you’re okay with having no atmospheric light during twilight hours.
Using a two-sided material is the most expensive, but also the most reliable solution.
Appreciate it.