Este artigo tem uma boa análise da aversão a Orcs à luz do sol.
A luz do sol parecia ter mais efeito psicológico sobre os orcs, assim como as trevas de Mordor deprimiam o moral de todos os homens que ali entravam. Muitas vezes ao longo dos Silmarillion Orcs lutam sob a luz do sol, alimentados pelo ódio de seus inimigos ou pelo medo de seus mestres sombrios.
Então, em certo sentido, eles odeiam a luz e preferem não estar nela, mas quando forçados a isso, podem viajar e lutar normalmente. A principal passagem de interesse da LOTR, quando Aragorn e companhia estão perseguindo os Uruks:
The orc-band began to descend a narrow ravine leading down into the misty plain below. Merry and Pippin, separated by a dozen Orcs or more, climbed down with them. At the bottom they stepped on to grass, and the hearts of the hobbits rose. ‘Now straight on!’ shouted Uglúk. ‘West and a little north. Follow Lugdush.’
‘But what are we going to do at sunrise?’ said some of the Northerners.
‘Go on running,’ said Uglúk. ‘What do you think? Sit on the grass and wait for the Whiteskins to join the picnic?’
‘But we can’t run in the sunlight.’
‘You’ll run with me behind you,’ said Uglúk. ‘Run! Or you’ll never see your beloved holes again. By the White Hand! What’s the use of sending out mountain-maggots on a trip, only half trained. Run, curse you! Run while night lasts!’
Then the whole company began to run with the long loping strides of Orcs. They kept no order, thrusting, jostling, and cursing; yet their speed was very great. Each hobbit had a guard of three. Pippin was far back in the line. He wondered how long he would be able to go on at this pace: he had had no food since the morning. One of his guards had a whip. But at present the orc-liquor was still hot in him. His wits, too, were wide-awake.