The body starts consuming less energy because sitting behind a PC for that long only causes one to not need much energy - for that reason, sitting for so long behind a computer will make your body go into a mode where it doesn't spend much energy. You also might tire your muscles through it as they need movement.
Yes, this is generally true. Staying still in general, even if one is sitting behind and computer and typing, is generally not healthy for the body, since the body requires movement in order to maintain proper circulation.
