Night is when the speaker misses someone most. Each stanza begins with the same line, "In the dead of the night," and the repetition works like a clock ticking through sleepless hours, or like a thought that keeps circling back. The "him" is never explained. We only know he is absent, and the speaker admits it plainly: "I know he is not here." What she sees is her heart trying to fill the gap.
The feeling is mixed. The memories "haunt" her and keep her awake, yet she lets them in anyway, calling the experience "bittersweet." His smile, laugh and embrace make her mind "swim," which suggests she is both dizzy and delighted. By the end the mood softens. She falls asleep smiling, comforted by the thought that seeing him again is only "a night's while" away. The poem moves from restless yearning to quiet hope, and the tone stays gentle, tender and a little wistful throughout.