Half-light can be forgiving—to the aging, to the vain, to the furtive philanderer—but in Trevor Nunn’s stunning, twilit, devastatingly good new production of A Little Night Music, it’s as punishing as the equatorial sun. Even at intermission, Nunn withholds full illumination, dimming the house lights to a low smolder. He’s clearly trying to induce an exquisitely heartbreaking case of seasonal affective disorder in his audience, and, fiendishly, he succeeds. “Perpetual sunset,” the chorus sings, “is rather an unsettling thing.” So is this beautiful re-Bergmanized revival of Hugh Wheeler and Stephen Sondheim’s elegiac sex farce (based on Smiles of a Summer Night), with its restored Nordic tilt, its bracing draughts of carnal realpolitik, and its ghostly blue ache of some-requited love. “It’s the latitude,” says the jaded ex-jade Madame Armfeldt (Angela Lansbury), explaining the madness of Scandinavians to her granddaughter (Keaton Whittaker). “A winter when the sun never rises, a summer when the sun never sets, are more than enough to addle the brain of any man.”