Izzy just turned 1 year 12/29 and she is only 2-1/2 pounds. She is just not a big eater and runs around like crazy. She is active in the morning and then around 12-1pm (on days home from work) she will sleep. Then up and going again until 9pm where she just konks out. She does sleep more than a bigger doggie and that's also what the vet said. They are tiny and it doesn't take much to make them tired when they run around the house chasing cats or outside chasing squirrels.
I've also found that the more daylight the more energic she is and once Winter set in with the time change she slows down when the sun goes down. Can't wait until Spring.
She is supposed to eat a 1/2 cup of food a day and I'm lucky if she eats 1/4 cup per day so the vet also recommended giving her more healthy treats inbetween.