This is the final stage of your field testing, and perhaps the most difficult, but when it is done you will know the overall makeup of the mattress that works best for you. You could be your own custom mattress “designer”. This step may take a little longer than the others so make sure you set aside enough time so you can do this in an unhurried environment. Sleep testing needs to be a "mellow" experience that brings an easy smile to your face. Remember that what you do today, along with the other steps, will have a big effect on the third of your day you spend sleeping and on how you feel in the other 2/3 as well, every day for many years. Take your time. There is no pressure allowed :).
So far, you have narrowed down the overall feel of the mattresses you like, you have a good idea of the thickness and type of support materials that work well for you (and if you don't, our forum is there to help you narrow this down and complete step 2), and now it's just a matter of deciding on which support core underneath in combination with your comfort layers gives you the best alignment. In this step you will be specifically testing what is underneath comfort layers that are similar to those you have chosen in step 2 to see how well they work together for alignment and add or take away from the overall feeling of your mattress. You have probably already developed some support layer preferences as a side effect of your previous reading or testing as some of them may have felt better or "more supportive" than others. These preferences would be a good place to begin.
As a reminder, your 4 realistic choices of support layers are Innersprings, Polyurethane foam, Latex foam. or waterbeds (unless you are seriously looking at airbeds in which case we would strongly urge you to do some research and read the information here to validate some of the claims you have likely heard). Narrow these down to two as early as possible in your testing if you haven’t already done so or already have a likely favorite type.
For this step you will need a pillow that is suitable for your sleeping positions (thicker for side, medium for back, and thin for stomach) and a yardstick or a broomstick handle (and no you won’t be pretending to be a witch in the store :). It is also preferable to take someone with you ... preferably someone you trust or your sleeping partner ... to help you check your alignment. If they are a sleeping partner they too should be following the same steps as you. Failing this you will need some help from the salesperson so make sure you trust that they both can and will give you exactly the information and feedback you are looking for.
In the store:
Choose your first "candidate:
First choose a couple of mattress candidates. If you tell your salesperson specifically what you are looking for in terms of comfort layer thickness and type, they should be able to help you find good candidates. Requests for help along the lines of...
"I'm looking for a mattress with about 3" of latex (or memory foam or a 3" softer comfort layer etc.) and I'd like to test it on different support cores to see which one keeps me in alignment the best" will work very well. They will likely be very impressed with what you already know and will be less likely to steer you in the wrong direction.
Use your pillows:
Make sure you use your pillow (or ask for one from the store for your testing) that will keep your head and neck straight in your sleeping positions on each mattress you test as this will affect other areas of your body more than you may realize. In general terms a pillow should be thicker for side sleepers, a little thinner for back sleepers, and very thin or none at all for stomach sleepers. Filling in the "gap" between your head and shoulders with your pillow ... especially for side and back sleepers can make a big difference in how you sleep and in the alignment of your neck and head and other areas as well.
Imprint your shape:
It will also help just before you lie on your first mattress to stand up straight (with good posture just like your mother used to tell you) and have your "helper" look at the "S" shape of your spine from the side to "imprint it" in their mind before you lie down on a mattress. Repeat this along the way as they feel it is necessary as a “reminder”.
To get a good sense of alignment on each mattress, you will need to lie on each serious candidate long enough to completely relax so that muscle tension is not holding up your spine. A few minutes spent relaxing before you begin testing is a good idea. Make sure you spend at least 15 minutes on each mattress you are seriously considering with a good part of this in each of your sleeping positions.
Testing for alignment:
On your back and stomach your spine should be supported in its natural "S" curve and on your side it should be completely straight ... including your neck. To test for "straightness" on your side, your partner can use the yardstick or broom handle as a reference and see how your spine looks in comparison. On your back and stomach they should squat down and take a look from the side to make sure the "S" curve is similar to when you are standing. If it seems that a part of you is down too low, try putting a thin pillow under that area to see if improves your alignment. If it does, you know that part of you needs to "come up". You're not looking for perfection to the millimeter since your body has some flexibility, but you also don't want any unusual "bends" that are clearly noticeable.
Testing for gaps:
Test for gaps (usually lack of lumbar support but any other "gaps" as well) in all positions by having your partner slide their hand under you in any position that looks like it may not be in firm contact with the comfort layer. It should be difficult to do. Make sure you test all your sleeping positions on each mattress as some support cores are very good at supporting you in one position but not in others. Other types of cores have a greater range of response and can keep your spine aligned in multiple positions. Be careful about confusing firmness with support.
On your side:
Lie on your side if you sleep in that position and try to sense any strain along your back or neck. Lie quietly and feel what your body is telling you. Pay particular attention to any gaps in this position as well as whether the shoulders sink in deeply enough.
On your back:
Now lie on your back if you sleep in that position. Once again try to sense any areas of strain along your back or neck by lying quietly and listening once again to your body. Pay particular attention to any gaps in the lumbar area, how deeply the pelvis is sinking down, and whether it seems the upper back is being held up too high.
On your stomach:
Finally lie on your stomach if you sleep in that position. This is the most difficult (and most would say most unnatural) position to accommodate. Make sure you have a thinner pillow. In this position it is especially important to completely relax and quietly sense if you feel any areas of strain or if you feel an urge to tense up to support yourself. It would be a good idea to spend a little longer on a mattress if you sleep in this position to make sure your spine has completely relaxed. Pay particular attention to any hyperextension of the back here. Have your partner look very closely at your spinal alignment and pay attention to any areas that sink in too much or not enough to support the natural "S" shape of your spine when you are standing.
Firmer or softer:
A mattress core that is too firm can lead to "gaps" in the recessed areas of your body. This means poor support. You need to "sink down" deeper in one or more areas. On the other hand a mattress core that is too soft for you will let parts of you sink in too far (especially your hips) and cause strain somewhere along your spine as you will not be sleeping in your natural "S" alignment. You need a firmer support layer that will bring the "offending" part up. This too is poor support. Each mattress you test should move you closer to knowing what works best for you, and what you should test next.
Often the hips of a side sleeper tend to sink in too far and need firmer support while the shoulders tend to not sink in far enough and may need something softer to stay aligned. Try changing the ILD or firmness of the innerspring. It is sometimes surprising that because of the different properties of foam or innersprings that firming up an entire layer to bring the hips up higher may still allow the shoulders to sink in enough even though you may think it wouldn't. In the same way softening up the entire layer support layer to allow the shoulders to sink in deeper may surprise you that it doesn't allow the hips to sink in as deeply as you thought it would. All of this is connected to progressive resistance and other factores of each support material.
Don't rule out categories too soon:
Be careful, at least at first, not to rule out "categories" of support cores such as "I don't like an innerspring core or I don't like a latex core) since different firmnesses in each material will be completely different and different types of latex (Dunlop or Talalay) and different types of innerspring (Bonnell, Pocket coils etc). In the beginning think in terms of "too high/too low" and "firmner here/softer there and general characteristics, then start narrowing it down to specific types. Rule out categories when you know for certain that is not the direction you wish to go.
Zoning for tough situations:
If you are having difficulty getting alignment just right because it always seems that part of you is sinking in too far or not enough, then you may wish to try zoned mattress cores (different firmnesses in different sections). If it seems that your hips are always sinking down too far, then a mattress with a firmer zone underneath your hips is well worth trying. If it seems that your shoulders are always not sinking down far enough, then a mattress with a softer shoulder zone is likely the answer. If it seems that both your hips and shoulders are not sinking in far enough and there is always a "gap" under your lumbar, and you don’t wish to use a thicker comfort layer (perhaps one of your positions is a stomach sleeper) then in these rare cases a zoning system that is softer under both your hips and shoulders and firmer under your lumbar (not under your hips) will lower your lumbar and allow it to contact the mattress for support. (this can be checked again with sliding the hand under the recessed areas which should be very difficult to do). Be careful with zoning to make sure it is appropriate but don't hesitate to use it if it's really necessary.
When you and a partner can’t agree:
If you sleep with a partner and you both have widely different preferences and cannot find a mattress that offers both of you support, then each of you should test your mattresses individually and another form of zoning is appropriate. This is side to side zoning. This allows you to choose different comfort layers and different support layers individually. They are widely available in specialty shops, local or regional manufacturers, or online if you do not find a place that offers them near you. The forum can help you with this as well. You do not need to get separate single mattresses and put them together any more unless you wish to. This solution has certainly saved more than a few "arguments" about who gets preference.
If you cannot find the correct choices or zoning in the store you are in, go elsewhere and start where you left off. Let the sales person know that you want a "plush mattress with approximately 3" of latex or memory foam (or whatever you have narrowed it down to) on top of whatever your support choices are so far and that you are sinking in too far in your hips and couldn't find the proper zoning. What do you have that may help me? They will certainly be impressed with what you already know and once again far less likely to mislead you or waste your time.
Some helpful tips:
Although I question the real value of air chambers as a permanent support layer (see my previous post about airbeds), finding a good airbed in a store that has comfort layers similar to what you chose in step 2 may provide you with a chance to test out more extreme customization in zoning (within each side and side to side) that the two of you require so you can discover what combinations of support work for both of you with your chosen comfort layers. They are readily available with many different comfort layers and provide a good "testing ground" when all else fails (although they may not be the best value in a purchase compared to a correctly zoned innerspring of latex core which provide a wider response for different sleeping positions).
Now you are done:
It's probably been a long day but you've done the most difficult part and now your field testing is over. You have a clear idea of the type of support system works the best for you. This in combination with knowing your preferred comfort layers (from step 2) and preferred overall feel (from step 1) means you are ready to go home and evaluate exactly what you need in your perfect mattress. You should now be able to write down a very close description of how it is made. All that's left now is deciding between different options based on quality/durability and price/value and deciding where to buy your mattress. As always, if you need help, our forum is always available to help.