What could cause a dull tooth ache at night?

Pulpitis. A dull, nagging night time tooth ache is usually caused by a tooth pulp that is inflamed. Your dentist should be able to diagnose the problem. If that is indeed the problem then you will need to have a root canal done on the tooth. There could be other causes but it is important to see the dentist to determine the cause.
Bruxism. Many things could cause this and i would recommend you see your dentist for a good check up and official answer. However, often we clinch our jaws and grind our teeth at night and this can cause dull pain in the teeth.