Making Nighttime Light Level in Custom Dimension Low

I've noticed that during the night phase in my custom dimension, light levels remain around 14. This prevents any mobs set to "Monster" to spawn. This is a problem, because I want the mobs to only spawn at night, but nighttime light levels are too high for them to spawn. If anyone has any idea on how I could make sure light levels were low enough for them to spawn during the night, please let me know.