Chihuahuas don't have the same need for exercise as, say, a border collie, and though they can be pretty hyper, they also are kind of a lazy breed lol! Plus given their size, just running around the house after their humans, playing, jumping on and off the couch, give them a lot of exercise right there.
Mine pretty much sleep all day when I'm at work. They sleep again if I go out at night. Then they sleep the night through. On the other hand, if I'm home and doing stuff like on the weekends, they'll be up and interested and involved, all day sometimes, but MAN do they crash when it's time!
Chis just seem to be VERY adaptable.