Most people sleep with their feet pointed down. This tightens the plantar fascia and is one of the reasons for morning heel pain. A night splint stretches the plantar fascia while you sleep. Although it can be hard to sleep with, a night splint works well and does not have to be used once the pain is gone.
There are several styles of night splints available including tension based, rigid and dorsal styles. All work, but personal preferences are different. Some people struggle to sleep with rigid devices, where tension based can be bit too aggressive.