Different Tamagotchis are programmed to sleep during different times of day/night. Generally speaking, Tamas will fall asleep from 8-11 P.M. and will wake up from, well, 8-11 A.M.. If this isn't the case, you may have set your device to the wrong time. I'm definitely guilty of accidentally setting the time to A.M. instead of P.M. and vice versa.
Also, your Tama's weight is totally normal! Assuming your Tama is in the toddler stage, its minimum weight is 10 pounds. Essentially, you're right where you want to be. ^^