Policies taking longer to run during the evening

I have a situation where certain policies created are now beginning to take considerably more time to back up.

There have be no changes to the policies, nor to the client bpstart scripts however during the evening it is taking 5 times loner to complete than it odes during the day.

The version installed is 7.0.1, and we backup to staging areas on a storage subsystem before writing to tape. There is ample space on the storage.

The only thing that is different is the time. 

This is quite bewildering, so any advise will be greatly appreciated. 

Thanks