what is the frist thing thomas notices after he hears theresa say she has "trigger the ending"?

Thomas wakes to a weak, dim light. He thinks he must have woken early, before dawn, but then he hears shouting. As he studies the sky beyond the canopy, he sees it is a dull gray instead of the pale light of morning. Looking at his watch he sees that it is a full hour past his mandatory wake time. The sky is simply gray. The sun has disappeared.