Hi, ive done vast studies in speech and language therapy especially in paediatrics. Throughout the programme, ever since the first week the effects of dummies and thumb sucking has been hightlghted to us. I dont want to make you panic but I shall tell you a few of the things that we have been taught. Thumb sucking and dummies do exactly what they say on the tin, they keep your children quiet! At this stage children are just starting to explore language and sounds, and by having a dummy or a thumb in the mouth can affect the developemnt of correct speech sounds, and also the oral muscle development. However you say she only tends to do it when she is tired which is naturel. If it becomes a regular thing and she starts to do during the day or through vocalisation, then try to ween her off it. Hope this helps!