It seems to be weekly occurrence at the moment but Sony has said that the PSN will again be down for maintenance  from next Thursday (1st March) at 4pm until 7AM the following morning.

Unlike the usual procedure no online services will be available during the downtime which includes online gaming.

“Users who are already signed into their account on PlayStation Network before the start of the maintenance will not be able to stay signed in to their online session and will be signed out at the start of the downtime period. Users who attempt to log into their account after the maintenance begin will be presented with the site maintenance notification page.”

Source: PS Forum