They are triggered by light levels I'm sure. Weekdays I start cooking around the same time and the piggies are asking for veggies and munching on their hay and pellets - all very active. If there is a delay - like a holiday or I'm a bit poorly, at least one will come out to the kitchen at that time looking for me and stare through into the living room like, "How come you're just sitting there? It's dinner time!"
Sometimes I get up in the early hours and make a cuppa but they know they're not going to get anything. There are quiet murmurings but nobody wants to get up before dawn. They also don't bother if floorboards creak in the night as we go to the bathroom. Any time from 6.30 is fair game for running out squeaking and if I'm not up by 8 they'll squeak so loud they wake me up anyway! These times shift a bit between summer and winter so I'm sure it's the sun that triggers it - they're indoor but there's lots of natural light.
they know who is boss - them (the way it should be 