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.