Bai's poem is only two lines long, but it captures a feeling most people know well: lying awake at night because someone won't leave your mind. The title, "Sleeping," sets up what the speaker is supposed to be doing. The first line, "I should be sleeping," sounds almost like a gentle scolding, as if the speaker knows the hour is late and rest is the sensible choice.
Then comes the turn. The word "But" cuts against that sensible voice, and the second line admits the truth: "I'm up thinking about you." There's no drama or long explanation. The plainness makes the confession feel honest and a little vulnerable.
The speaker seems caught between what they ought to do and what they can't help doing. We don't learn who "you" is, or whether the feeling is joy, longing, or worry, so the poem leaves that open. The mood is quiet, intimate, and slightly restless, like a whispered thought in the dark.