Yes and NO. Yes of course, it can be bad for you. You sit in one place for hours, staring at images. However, if you remain hydrated throughout (by drinking water, no beer or soda) then you will be concentrated for longer. Every half an hour, stretch your legs, stand up, change position on the sofa. Take a break for an hour or two every two to three hours. Do not play all day, no matter how tempting, you will become addicted. It is likely that someone who played all day would put on weight because they aren't going to get any exercise (perhaps the Wii might change that!)
Also, it is impossible to eat a proper meal whilst playing, which means either digestion will be affected or you are snacking or eating things which can be eaten quickly with the hands, again not so good.
Of course if you follow my advice, you will find there are few if any ill effects.
Yes. You are suppost to take 1 hour breaks in between your playing
Yes, I Have been on the sims too much and I have now got bags under my eyes and my eyes (I had this when I was a baby) watering too much