There's a scene in Insomnia where Al Pacino tries to get some sleep in his hotel room, but since it's daylight 24x7 in Nightmute, he can't get any sleep. So he papers all the windows so the light can't come in, but it still keeps streaming in through the gaps.
Background: Pacino plays a cop who has been called to Nightmute, Alaska on a special assignment, to investigate a homicide. Back home, there's an ongoing Internal Affairs investigation that might well lead to his doorstep. And to complicate everything, he sets a trap for the killer, and while chasing him through the mist in a forest, he shoots his partner accidentally. He covers it up by blaming the killer, but the killer knows the truth and blackmails him.
The movie is all allegory: Pacino is in a place where it's always day, and he can't get any sleep. But the condition referred to in the title is not of his body, but of his conscience. And I thought the scene in the hotel room brilliantly encapsulated it.
You don't see there, a man who can't sleep because the sunlight is streaming through his hotel room window. You see there, a man who can't sleep, and wishes it were because the sunlight is streaming through his hotel room window.