Can you use FIR amethyst mats at night on a bed, and how comfortable are they?

You can use one at night for up to 12 hours. We suggest you use a slightly warm, comfortable temperature (the lowest setting, up to 45°C/120°F). Never sleep with the mat set to a high temperature. 

As for comfort, it is as comfortable as stone pads could be. It’s not the same as a spring mattress or a foam mattress. Most people enjoy lying down on this pad but don’t expect the comfort of a soft mattress. It’s different.