Milk supply and sleeping through the night

My son just started sleeping through the night (from 6-8 hours at a time) this week. I’m used to pumping every 4 hours during the work day or feeding on demand when I’m with him. At night though I wake up sore and full but he usually only gets one side before falling asleep again for 3-4 hours. Should I pump the other? How do other people do this? I’ve noticed my milk supply lessen during the day even though he has also started drinking more milk. Any ideas or tips please?