Why did saya fall asleep again at the final ep of blood+? is it because she failed to get transfusion?

Well just like many people have said... saya is only able to stay awake for like 2 years at a time... then she goes into a 30 year sleep i believe... she starts to feel sleepy around the later half of the series... since her job is done she is unable to resist anymore... so she falls asleep...