"Die Blümelein, sie schlafen" is the first line of the German lullaby "Sandmännchen" (Sandman), from Anton Wilhelm von Zuccalmaglio's collection Deutsche Volkslieder (1840).
The melody is based on a French song from the late 1500s which was also used for the Christmas carol "Zu Bethlehem geboren" (1638) to a text by Friedrich Spee.
A sandman is a mythical figure in German and other European folk tales who sprinkles sand in the children's eyes at night, making them drowsy and inspiring beautiful dreams.
Sandmännchen kommt geschlichen und guckt durchs Fensterlein, ob irgend noch ein Liebchen nicht mag zu Bette sein.
The sandman exits from the room, my darling sleeps so well, his eyes are shuttered firmly, quite tightly locked the lids.