After every few waves, night falls and the whole map turns dark. During night waves, most turrets cannot attack unless the enemies are lit by a light source.
Wave 5, 11, 36 are night waves.
Here are a variety of ways to defend against the night.
1: Use the delete button to dig 2 blocks under the ground. Build a power generator there. Cover the block above it with any block, preferably wood. Now dig holes for lights around the area of the generator. This makes perfect bait blocks and night vision as well! ~Hexapheonix.
2: Over time, work on crafting detector mods for your favorite turrets. This mod allows them to see in the dark.
(Post your defenses against the night here if you wish to share)