Bai's poem is a single line, and that's part of what makes it hit. The speaker is awake at four in the morning, the hour when the house is quiet and nothing distracts from whatever is weighing on you. In that stillness, one feeling takes over: missing someone.
What stands out is the word "tired." We usually think of missing a person as sharp or aching, something romantic or dramatic. Here it's worn down into exhaustion. The speaker isn't saying the longing is new or sudden. "Of missing you" suggests it has gone on for a long time, repeating night after night until it has become a burden.
The mood is quiet, bare, and a little hopeless. There's no anger and no plea for the person to return, only an admission of weariness. The short length mirrors that: the speaker has little energy left for more words. The title leaves us to imagine the sleeplessness behind it.