Instructions
Open up your custom-made map and click on the "Triggers" button in the editing menu. Click on the "Make a New Trigger" button on the menu.
Create an event that will cause your trigger to activate. Pick any event you like, such as "character moving into view."
Select the "Actions" menu, click the "Environment" submenu that opens up, and then select "Lighting."
Change the intensity level of the light by typing a numerical value in the "Level" text box. Select the "day," "night," "sunrise" or "sunset" quick set options to pick a specific time of the day to emulate.
Set the timing of the light trigger by inputting the number of minutes and seconds you want the light to stay on in the "Timing" text box. Save and run your map to check the trigger.