Tokyo Subway will begin to sell 24hours Ticket instead One day ticket.
You will be able to take the train for 24hours from the time buying a ticket.
One day ticket which is the active system, is available from first train to last train on the day.
I think it will be useful especially for tourists.
Tokyo Subway 24hours ticket : 800yen
Only Tokyo Metro 24hours : 600yen
http://headlines.yahoo.co.jp/hl?a=20160204-00010003-norimono-l13