See

https://www.nature.com/articles/s41598-024-77683-1

That kind of module can be really cheap

https://www.waveshare.com/hmmd-mmwave-sensor.htm

and is starting to replace Passive IR sensors

https://en.wikipedia.org/wiki/Passive_infrared_sensor

I can imagine one of these on the ceiling above your bed being an ideal sleep monitoring system.