I swear by a CD on repeat.......we use Peacebaby for Olivia and Charlie and Lexie and it is 47 mins long and has the 60 beats per minute thing (like a heartbeat or something?) and it seems to "cure them" of the 40 minute wake up.

They still go through 40 minute sleep cycles, but I am sure their subconsious hears the tinkle tinkle of the CD and they drift off again happily with no crying.......