J.G.
My daughter is 20 months old and still sucks her thumb. She isn't sucking her thumb more often than she used to, but she never did suck it at night only. She has always sucked it on and off during the day depending on her mood--bored, tired, overwhelmed/anxious.
As long as the thumb sucking doesn't interfere with your daughter doing other things and playing with toys so that she can learn important developmental skills (talk, stack blocks, push cars/balls, etc.), I wouldn't worry about it too much.
I've noticed that as long as my daughter is staying busy and having fun, she doesn't suck her thumb as much (as long as she isn't tired or overwhelmed) so perhaps you could try to keep your daughter busy with activities that require her to use her hands.