Question about Withings Sleep Analyser

That should work. What might help is to know that the mat is only about 60x20cm and is placed transversely under the chest area. At first I had also wrongly assumed that the mat is placed quasi lengthwise under the entire body.

This analyser should be placed under the mattress, level with your heart. As long as it doesn't move, there's no reason why it shouldn't work. A small piece of double-sided tape between the analyser and the slat underneath should prevent it from moving when handling your list head. It's very sensitive, I have a 20 cm mattress and it took me a while to realise that the naps detected during my absence were my cat's naps in exactly that spot. :-D