Zebra finch sleeping problems?

I have had a pair of zebra finches for around six months now. It is a male and female pair. They have been very happy and content until about a week ago. The male one has stopped the female from going anywhere near the nest they have. During the day he let's her go to and from the nest freely but at night he suddenly turns and doesn't want her in the nest when he is sleeping. Does anyone know why he is doing this or know how to stop this behaviour?