maybe Radon gasses? they have testers/ alarms you can get. like smoke alarms.

also the heating ducts can get full of gunk and maybe need cleaned out.

Is the home near large power lines? Or cell phone towers?
read about EMF fields.
Electromagnetic Fields -
http://people.uwec.edu/piercech/EMF/

there's much more info on it this is just one of the first sites I came across.