Restañar literally means "to staunch" or "to stop bleeding." It's a vivid and somewhat uncommon verb, more often found in literary or poetic Spanish than in everyday conversation, which makes it stand out in the lyrics.
In the context of the song, the phrase "noches de restañar" ("nights of staunching") uses the word metaphorically, suggesting nights spent trying to heal an emotional wound. It's a striking, almost medical image applied to matters of the heart.
Guasones's "La Vida Es Así" [Life Is Like This] is a really honest look at life and relationships. The singer talks about how a song can bring back memories, and how a drink can help you forget a heavy drug. It's a journey that brought him to where he is now, but he knows who he is. He wonders if it was just time that made things the way they are, not some grand, dramatic shift from a "celeste" [sky blue] to grey.
He tells someone, "yo te quiero con mi corazón pero lejos" [I love you with all my heart, but far away]. It's a blunt truth: "la vida es así" [life is like this]. He urges them to break free from a "rutina que te mata" [routine that kills you] and return to the "noches de resaca" [hangover nights]. He misses getting lost, and he sings about the things that happen to him. There's a moment of frustration when he says, "te pones a llorar piense que sé es que deberías poder escucharme solo una vez" [you start to cry, I think I know, you should be able to hear me just once]. Even if someone blames him and calls him the worst, he knows they still want him, even if it's "muy muy lejos" [very, very far away]. It's a song about accepting life's messy reality and the complicated feelings that come with it.