I think darkness is such a trigger because there is nothing (or not much) to visually distract your mind. Therefore, the darkness allows your mind to wander internally, and with ptsd being the forefront of everything, the brain will wander into places we'd rather forget...even if it's subconscious. I find myself zoned out in the darkness, making plans of how fast I could grab my gun if Justin were to break into my house, or different escape routes, or how I could subdue him and get my daughter and escape, etc. My mind is consumed with thoughts like these, when my rational mind has to sometimes step in forcibly and tell me, "He is 1,200 miles away, he does not know where you live, what car you drive now, where you work, nor has your phone number even". This is reassuring at times, but I definitely sleep with lights on!