Goal: make hardwood slats for a bed thin to flex, w/o degradation
IKEA sells slatted wood bases for beds. The Luroy model is the most basic where “16 slats of layer-glued birch adjust to your body weight and increase the suppleness of the mattress”.

I’m not quoting them as an advertisement but just to give an example solution. (Although, you should consider whether the double/full at 52 3/4” wide could be used or modified to suit your need.) 

Perhaps you don’t want to tackle layer-glued wood to start with a bend. Straight hardwood works but is going to be a little less springy since you will have to make it thicker to avoid rolling towards the center.