How much of basic activities should I be logging?

Hi - I'm a new member hoping to use the site to maintain my current weight after successfully losing 20 lbs! I noticed that there are calories automatically entered when I get up in the morning and in reading through the new member forums I realize that they are calories I've burned by just 'existing'. That's great, but I noticed that sleeping, working, walking, etc. are options on the activity list. How detailed do I need to be in logging activity? I work at a desk all day so I have 8 hours of light administrative work tagged as an activity, as well as 20 mns of eating, etc. when I have breakfast & lunch - is this necessary?