To set nightmode in Sequencer you first need to track BP_Nightmode in Sequencer. Then you need to Track → Events → Trigger
Then you need to add a new key to the timeline. Right click on the key and set the event to “set NightTime” you can use the search box to find it.
Once you have added the event you will need to go back to the key and right click on it to get to Properties where you can then Check the “Call In Editor” Checkbox. This will allow this trigger to work in the Editor.
Now you can run your sequence and it will trigger the nightmode in the editor. It can take a couple seconds for the Lumen to adjust to nightmode so I suggest adding your key a few frames before the start of your sequence.