Early in the breast augmentation recovery, your pectoralis muscles are not yet "accustomed" to being permanently stretched by the presence of submuscular implants. At night you do not swing or use your arms very much (except for minimal movements as you shift your body and/or dream), and your pectoralis muscles tighten up to try to return to their previous unstretched state. Morning firmness results.
As you wake up and move around, this gets better, at least if you have little muscle inflammation or bruising, which of course improves as your recovery progresses. Too much movement causes bleeding, so don't over do it, and follow your surgeon's activity restrictions scrupulously!
Your muscles will reach a new steady state as time goes by (every patient is different), and this morning firmness will go away permanently. Fear not, this is normal! Best wishes! Dr. Tholen