Hi Meri831,
As zexpress and “the Saatva guy” mentioned … the most common reason would be a mattress that is too firm but it’s certainly not the only reason for the “symptoms” you are describing because it can also be a result of your sleeping position and it could also be a pillow issue as well.
Did you read the comments at the beginning of this post (and the post it linked to about mattress "symptoms)?
There is more information about Saatva mattresses along with some of the other simplified choice online mattresses in post #2 here and in post #1 here and the rest of the topic and a forum search on Saatva (you can just click the link) will bring up more comments and feedback about them as well.
While it’s not possible to predict how any mattress will feel for someone else with any certainty because there are too many unknowns, variables, and individual preferences involved … there are certainly other “simplified choice” mattresses that use better quality and more durable materials and I would also be cautious about buying any mattress based on the “marketing information” that is on their website.
If you are looking at online options then outside of the simplified choice list the mattress shopping tutorial also includes several links to lists of many of the better online options I’m aware of (in the optional online step) that include many different types and categories of mattresses in a wide range of budgets and firmness levels that would be well worth considering.
Phoenix