I know it is because Betty the guinea pig tells me it is.
9 o'clock is usually the time she gets her daily dose of veg.
She lives indoors in the corner of our living room. We hear a few little wheek wheek noises coming from the corner at about 8, then by 8.30 the wheek wheeks have turned into wheek wheeks, and by 9pm they are full on wheeeeeeeks!
We rescued Betty from the RSPCA a few years ago. She is now rather old and is diabetic and almost completely blind (although she does always seem to manage to run to her veg bowl when she needs to) so she has had quite a tough little life.
We do tend to spoil her, quite a lot...