There are two, and only two, responses we get when people see Ginny sucking her thumb.
1. Oh how cute! She's sucking her little thumb!
2. Uh-oh, you've got a thumb sucker.
Now with both my girls (Joanna was and still is a big thumb sucker) I have put myself in the first category and added a "Thank heavens!" In my experience, thumb sucking=good sleeper.
There is many a dire warning that comes (unsolicited) from those in the second category. It will mess up their teeth (no problems yet with Joanna), it will interfere with eating (again no problems here). It's such a hard habit to break. Hmm. Now we haven't come to that bridge yet, but I have hope, since Joanna has kept the thumb sucking to times of distress and sleepiness.
And really, when it comes to getting good sleep during those first years, you'll take anything that will help. When Ginny first sucked her thumb and followed her sister down that path, I swear I heard the Hallelujah Chorus playing softly in the background.