Should I worry about sagging joists?
A properly sized floor joist should never deflect. Is your bed a water bed?. Joists are supposed to be sized to handle a minimum of 60 to 80 pounds per square foot, which is actually a lot, since loads are distributed across the entire span.  If you are seeing ridges, cracks, or movement under load, you have a problem.  I have seen joists wain, crack or even fracture at a fault in the wood grain.  If this situation has gotten worse, or happened suddenly, I would seek a pro to take a look. Don't want to scare you, but this is not normal and warrants further investigation. This is not a job for an amateur.