It is midnight, and the poet awakes and looks upwards to the heavens. But no star is shining. No comfort. No pathetic fallacy "kein Lichtgedanken mir Trost gebracht". The only sound he hears is his own heartbeat. "Nahm ich in Achtn Die Schläge meines Herzens; Ein einz'ger Puls war angefacht". Try this sometime, it's frightening to realize that all that keeps you from dropping dead is a piece of muscle. We don't know what metaphysical "battle" the poet speaks of, except that he knows that it's not up to him; he hasn't the power to change things. "Kämpft' ich die Schlacht, O Menschheit, deiner Leiden; Nicht konnt' ich sie entscheiden Mit meiner Macht" So he puts his faith in a higher being, ie God, "Hab' ich die Macht In deine Hand gegeben! Herr über Tod und Leben Du hältst die Wacht!" That's the breakthrough, being able to stop trying to control what cannot be controlled. To accept that there are things beyond our comprehension. Zen. In Rückert's case that means Christanity but it applies to all faith. Don't claw and scramble, chill. Faith gets a bad press these days because it's been hijacked by intolerant control freaks, who don't understand what it's really all about.
In the poem, Rückert repeats the words "Um Mitternacht" at the beginning and end of each stanza, with the insistent regularity of a heartbeat. The song has been orchestrated, but the version for piano song is more private and intimate as befits the situation. Then, when the poet puts his faith in something beyond himself, the vocal line rises "HERR!" Notice how the piano line falls evenly, like an inner pulse. I much prefer hearing a baritone in the part, because most Mahler songs are written for low voice and the poet's perspctive is masculine. Christa Ludwig is good and Gerald Moore is too, but YouTube is pot luck. Ten years ago Matthias Goerne was singing this, so wonderfully that a noted Mahler devotee asked that the off air tape be played at her funeral. Goerne makes the "Herr!" ring out with intense depth, stretching the word so it fills out, like his whole chest (and heart) reach out. . Incredibly moving. He's still singing it, and no doubt with even more depth of experience, but he doesn't get the marketing fuss he deserves, but I haven't heard him sing it recently.
For the full text and translation, please see Emily Ezust's Song and Lieder Texts Page here - fantastic resource.