asleep.
It's been a long time since Loki lay in bed at night, staring at the ceiling and waiting for either sleep to take him or for the eternal dark hours to give way to gray dawn. It used to happen almost every night, but somewhere along the line, he realized how much time he was wasting, waiting for sleep that would never come.
He presumes that Dr. Thatcher—the first of the short list of psychologists on call whenever an officer needs (or is ordered to take part in) therapy—would attribute the insomnia to anxiety and fears of losing control. Loki only presumes because he avoids shrinks like the plague after having to talk to a battery of them throughout his childhood, especially right before landing in a new foster home. He would disagree with the diagnosis, anyway. He's not afraid—if there's anything Loki is sure about, it's that fear has almost no place in his life, and when it does worm its way in, it's almost never for himself.
The insomnia comes, quite simply, from his inability—or unwillingness—to put his mind to rest when he knows there are problems to be solved. It just won't slow down, even when the rest of the state is peacefully sleeping.
He quit fighting it. These days, he doesn't go to sleep until he's utterly exhausted to the point of uselessness, and his stamina is impressive.
He drinks more coffee—at home and at work—than any cop he knows, taking it black because it's simpler and faster. He suspects it's a combination of the caffeine intake and the constant graininess of his eyes that prompted the start of the tic—the blinking. He tried to stop initially, even to the point of cutting back on coffee and trying again to get some sleep every night, but he soon realized that the tic didn't seem to signify weakness to anyone. If anything, people—especially suspects—became more on-edge around him. It's as if they sense the lack of rest, interpret it to mean that his inhibitions are lower, that he might actually cross lines put in place to protect them from guys like him (about that, they're kind of right).
He resumes drinking just as much coffee, stops trying for sleep that will never come, and spends his night hours instead researching, reviewing, and practicing keeping his brain sharp. The blinking stops bothering him, and eventually, he stops noticing it altogether.
Captain O'Malley refuses to let him pull all-nighters at the station when the cases he's working on aren't as time-sensitive, so he takes case files home and works on them there. He thinks O'Malley knows, but there's not much he can do—aside from making Loki go home, he can't tell him what to do with his personal life.
Still, every time Loki closes a case, O'Malley makes him take a couple of days off. He doesn't have anywhere to go, anyone to see, or anything important to work on, so the boredom is crippling and he can hardly wait to get back to work.
Still, the sleep he gets on nights when he doesn't have an active case is about the most restful he can expect.
A/N - On the topic of the blinking tic, Jake Gyllenhaal had this to say: "Physical things, like the twitch and stuff like that, that sort of began as an idea, early on. It really just felt right; I can't exactly explain why at first. And then, as I realized that he had so much going on in his mind, so many pieces of information to hold onto, and, emotionally, he was repressing so much, that sometimes that comes out in strange physical ways and attributes and tics. And I found that to be a way in which it was like an expression of this emotional state; he just couldn't emote, because it would hurt the case. It could hurt him in trying to figure out the case, it could cause great doubt. So he was just dealing with computing so much that it was almost like an overload. It was like a glitch."
With that in mind, I wanted to think about why Loki thinks the twitch developed. We can infer from him wearing the same outfit for days in a row, how tired he looks, and how obviously driven he is that he probably doesn't sleep nearly enough. I also don't think that he's one for a lot of self-reflection unless he has to do it in order to be effective, that therapists probably make him uncomfortable and he holds himself to seriously rigid standards so that he's never forced to go see one- standards that include squashing emotional responses before they can become a problem or before people start taking notice of his reactions. Of course, he's bound to slip every now and again once things build up beyond his control (as with the scene with the keyboard at the station), but for the most part, it works.
With those things in mind, I think sleep deprivation would be the most likely explanation that Loki is willing to accept and offer for this twitch thing he's suddenly developed after taking on the responsibility and the emotional burden of being a police detective.