Sometimes the ground can be too hard when praying on an ordinary prayer mat; and I end up with bruises on my knees. However, there is a more comfortable alternative...
Question: Is there anything wrong with praying on one's bed?
I've asked people about this, and they've expressed skepticism, but I don't see any reason why it's wrong. I'm guessing people think it's not the sunnah, rather than there being anything inherently wrong with it.
I've found it's also more convenient when traveling (not needing to carry a prayer mat) or in hospital.