Sony will be carrying out more PlayStation Network maintenance tonight / tomorrow (early morning). Fortunately, the downtime will occur between 1am and 3am GMT when most Europeans will be asleep.
Unfortunately its not so convenient for those across the pond in North America. During the downtime, all PSN related services will be inaccessible and gamers will see a maintenance message when attempting to sign-in to the PSN.
Sony also say there may be some “additional downtime” after the maintenance period for testing purposes.