The Lord Of The Rings: 10 Things You Didn't Know About The Nazgûl
4. The Ringwraiths Are More Powerful At Night
The power of the Ringwraiths came from their ability to inflict fear on their foes. Many times Tolkien alluded to the Nazgûl being far more dangerous at night, however. Although Peter Jackson featured many scenes of the Nazgûl in the daylight, this was not strictly faithful to the book.
They rarely moved around in sunlight, and were even weakened by it. Aragorn mentions that they're not likely to attack a house with lights and many people, and that they much prefer to trap someone in darkness. He insists they must only travel by day, on route to Rivendell. Gandalf also reveals that during the day, the Ringwraiths fled from him, only attacking with the coming of night.
Before Sauron launched his main assault on Gondor, he sent out a vast blackness from Mordor to cover all the lands in darkness. This was said to shield his servants from the sun, who thrive in its absence. It's often interpreted that this was for the benefits of the orcs, who hated sunlight, but given what we know about the Nazgûl, it's likely it was also to increase their powers.