Nights are always difficult for him.
In the dark, shadows hide demons and men, one in the same. Too often, his mind plays tricks and sleep only brings cryptic references to a past he'd rather forget.
Derek tells JJ and Reid that he's not afraid of the dark. It's a baldfaced lie - when they're forced to share hotel rooms, he'll pick Reid every time. He doesn't have to explain, doesn't even have to reveal that he still sleeps with a light on.
One night stands don't ask questions; he makes sure he doesn't stick around long enough for them to know the question even exists. But when Derek falls into a relationship with Reid, he assumes it won't be a problem.
For awhile, it isn't.
The first time they settle in for the night at Reid's apartment, it's three months in. He gets up to turn on the hall light, only to be pulled back into bed. "I only need the light on in new places and hotel rooms," Spencer mumbles, snuggling closer. "Pretty soon, I won't even need a light on at your place."
He doesn't sleep that night, just lays there listening to Reid's breathing, even and sure, wondering how much time he can buy before he has to brave the darkness just to be with someone he loves.
Apparently, his lack of sleep shows more than he hoped it would because Derek feels Reid watching him the next day, a quizzical look plastered to his face. He tries to ignore it, despite the fact that he knows Reid will search until he finds what he's looking for. They're on a case a week later, sharing a room, and when Derek flips the bathroom light on for the night, understanding flickers across Reid's face.
Most nights, it's Reid who turns on the bedside lamp, protecting him from the ghosts that linger, but he never questions Derek when he gets to it first.
He's sure that one day his lover's arms will be enough to lull him into a peaceful slumber, but until then, he's just glad to be with someone who's willing to defend him from the darkness.
