The PlayStation Network is to undergo maintenance tomorrow, Thursday September 8, between 5pm and 11pm UK time.
During that period PlayStation Store, account registration and account management services will be unavailable.
If you are already signed in before the beginning of the maintenance, you should be able to remain signed in and continue with your gaming session. Try to sign in after the work begins and you'll be presented with a maintenance notification page.
Sony said earlier this week it was surprised by the level of audience engagement with PSN following April's cyber attack on the service.