With most individual this is due to an overnight repair of the tissue.
This overnight repair often happens with the toes in a pointed position, think Ballet position. This is because it often gives an easing effect allowing you to sleep. However, it can be due to sleeping on a front or having a heavy set of covers pushing toes down?
This sustained position means that in the morning, during the first steps, it is painful as the repair is ripped out.
Night Splints can be ordered through the internet to stop the toes pointing. These are worn overnight and keep the ankle at 90 degrees. A good protocol is to use a foam roller to massage the tight calf muscle belly.
Do this every night for 1 week if no results report to the London Foot Pain Clinic for full consultations to rule out more severe damage to the tendon.