Hey Eredin,
In the Time of Day tutorial that I linked you to, there is a variable labeled “Night Threshold” which allows you to set the threshold in which the sun is turned off when it reaches a certain height. This variable is what you want to change and will be specific to projects since it is based of physical position of the sun disk relative to the origin of the map or level.
Night Threshold Variable
For my test bed project I have the variable set to 0.5. It turns off the sun right at the point in which it dips below my mountains and landscape. Keep in mind if you would like this effect to be perfect it is going to take some trial and error as to the value you will want for this variable.
Also be sure to add a ‘Skylight’ actor to your scene for your ambient fill light. This light actor is very powerful and has a lot of great features to help add realism to your project/scene as well. Let me know if you are still struggling to find an appropriate solution.
Cheers,