I dont believe level busts to be solely due to fatigue although I'm sure it MAY be contributary. Many other factors are also involved such as mishearing ATC instructions, correctly hearing ATC instruction and correct readback but inputing wrong level into autopilot etc etc....
The use of Mode S in the TMA has helped a lot in preventing potential busts and its a GREAT piece of kit. Only wish EVERY a/c in the TMA had it.
Spamcan