It's easier to sin in darkness than in broad daylight because darkness falsely leads you to believe that God does not see you. When you close your eyes to get ready to fall sleep, impure thoughts easily surface in darkness. And in sleep, you can get nightmares. In both cases, waking up to light restores your senses and assures you that everything is all right. Remain in the light of Christ always and you'll remain clear-headed and vanquish all fears.