Three short lines are enough to set up a very familiar kind of frustration: lying awake at night when sleep simply won't come. The speaker has done everything right. The shutters are closed, the blinds are shut, and the lights are off. Each detail adds another layer of darkness, like someone carefully building the perfect conditions for rest.
Then comes the final line, "If only I slept." It lands like a quiet sigh. All that preparation hasn't worked, and the poem suggests the problem isn't the room at all. The wakefulness is inside the speaker. The title, "Late and awake," confirms the hour is already far gone.
The mood is tired, still, and a little helpless. The poem never says what the speaker is thinking about, and that gap makes the restlessness feel even more private. The shutting-out of the world starts to feel less like comfort and more like being closed in with your own thoughts.