Heel pain is often worst first thing in the morning due to the overnight inactivity, which allows the muscles and tendons in the foot to tighten, especially the plantar fascia and the Achilles tendon. These tissues connect to the heel bone and are typically under a lot of stress during the day from walking or standing.
When you sleep, your feet are generally in a relaxed position and the fascia and other tendons slightly contract and cool down. Upon waking and taking the first few steps, these tissues suddenly stretch back to their functional length, which can cause micro tears or strain at the point where they attach to the heel bone, resulting in sharp, intense pain. This phenomenon is especially prevalent in conditions such as plantar fasciitis and Achilles tendonitis, and the pain usually decreases after the tissues stretch and warm up with continued movement.