This is a very common issue, and people have handled it in different ways since long before webcore existed, usually by adding different devices, such as a pressure mat for a couch or bed or a vibration sensor for a chair.
Some people find that putting a motion sensor under the desk down low catches enough movement from their feet and legs to keep the sensor active. 
You can see what other people have done by looking at the quick browse lists in the community-created wiki, looking down near the bottom of that page for the project reports by room section, and then selecting the lists for bedrooms. 
There are also a few on the list for sensor projects, including the following (the topic title is a clickable Link)