OP talks about "sitting, sleeping, using the rest room". Three different things, and at least two different rooms. One small room (an ultrasound sensor should do the job just fine), one much bigger.
For the bigger room you will need a combination of sensors such as motion detectors, heat detectors and/or CO2 detectors. Known heat sources (such as TVs) could be filtered out by simply detecting whether they're switched on or not. CO2 detectors of course will react slow and are affected by windows - sensors on the large windows can tell that they're open so you know the CO2 doesn't work. A window opening or closing, or a TV being switched on or off, can be used as yet another way to tell that there's someone in the room.
To check whether a person is using a chair or a bed, a weight sensor should work quite well. Unless it's a very small person, a 10 kg minimum would take care of most stray objects placed on the chair/bed.
You're going to need an array of sensors, and that way I believe you will get pretty good reliability. There will always be ways to fool the sensors, but those will more and more be only deliberate actions and not normal behaviour.