My 3 1/2 week old is the same, but she does it all the time not just when she's tired! People are always telling me she must be hungry, and I know as a newborn she needs to eat a lot, but not when she's literally just had a good feed. She won't take a dummy but she's starting to find her fingers, and when she sleeps I swaddle her but with a hand peeking out near her mouth so she can get it, or I sometimes give her a top up feed as you and others have described, especially in the evening. Sometimes she feeds to sleep and sometimes she doesn't. She definitely likes to suck though - arms, boobs, fingers, sleeves are all welcome!