What's a word for a light sort of sadness?

You know, like the feeling people get when they think about their childhoods, the type that makes them sigh and smile sadly. I'm not talking about deep melancholy that makes you weep silently, or the fierce pain that makes you scream. If you know what I mean, can you give me the word/words for this emotion? I'd been wracking my head for it for days now! thanks.