The PlayStation Network is undergoing maintenance tomorrow and as a result, gamers will not be able to sign in to the PSN between 4:15 p.m. and 9:05 p.m. GMT. The maintenance is global.
According to the PlayStation Blog, if you are already signed in to the PSN before maintenance begins, you should be able to stay connected. If so, you will likely have to sign back in once the PSN comes back up at 9:05 p.m.