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Somewhere, I can’t find the post now, but Phoenix explained the way that polyfoam breaks in when it is new, and why it softens up in the first couple of weeks:
The way I understand it, the foam is created of “cells” and as you lie on it, the larger cells of foam break down into smaller cells, creating a softer feel. However, I think after the first few weeks, after the larger cells break down into smaller cells, the foam stabilizes and should keep that feel. Given the decent density that Tuft and Needle uses in their top layer of polyfoam, I understand sagging shouldn’t be a problem for several years. And if the mattress does sag, Tuft and Needle has a warranty: From the Tuft and Needle website:
What if the foam forms irregular indentations and is getting body impressions?
This is unlikely to happen for at least 7-10 years. We consider increased softness such as body impressions or permanent indentations to fall under our 10 year warranty. If you observe a visible indentation greater than 3/4 of an inch (about the length of your thumb, from tip to knuckle) please let our team know so that we can help you replace the mattress.
The first week we had the mattress, I walked all over the surface once or twice a day. I am quite sure that this helped speed the break in process.
We have rotated our mattress once- so the head portion of the bed is on the foot portion (not “flipped”, but rotated). I am hoping this will lengthen the time the mattress lasts. I don’t know how often to do this, but planning every 6-8 weeks.