Hi jpgavin.
Welcome to our Mattress Forum 
!
Maintaining neutral alignment of the spine does not equate with a “straight line” spine in the shoulders area or even across the shoulders …You must sleep in a position that, allows as much as possible for the spine to assume it’s natural position (not a learned position that it is often a result of the body trying to compensate with any misalignment issues) The slight natural curvatures of the spine would allow for the spinal fluid to be unobstructed to lubricate and return into the spinal disks to maintain their 3 primary qualities (shock absorbers, ligaments holding vertebrae together, and flexibility to help facilitate the movement of the spine). A “perfectly straight spine” can certainly lead to pains and issues over time. The neutral alignment will facilitate the entire system of bones, muscles, tendons, ligaments, tissues, and joints to relax, reset, and repair any alignment issues or tensions accumulated during the day.
You did not mention what your primary sleeping position is or if you are trying to pinpoint the cause of any pains but I usually recommend to work closely with a chiropractor to assess and learn how neutral alignment would look for you. A chiropractor is trained to feel muscular tensions and manipulate and release them with subtle movements over several sessions.
Generally, to help with your assessment specifically on testing for alignment and its symptoms in all your sleeping positions there are several things that I usually recommend to keep in mind:
~ Try to sense whether your muscles are able to completely “let go” and allow the mattress to support your natural alignment rather than using muscle tension to keep you in alignment. This means that you can sense your body and muscles fully relaxing without a tendency for any area to be tense.
~ Next is to pay particular attention to any tension or discomfort (or even pain) in the areas where poor alignment tends to produce symptoms for you. This can be different for different people but is usually in the lower back or lumbar curve, and the upper back and neck where the spine also curves. Test in all your sleeping positions making sure to move slowly when you change position and stay relaxed. Bear in mind that minor discomfort when you are testing can be amplified when you are sleeping for longer periods of time.
~ Next is to make sure that all the inner curves of the spine are filled in so that there are no “gaps” in-between your body and the mattress. It should be fairly difficult to slide your hand under the lower back or waist (if the mattress is too firm then this area will not be filled in well enough and sliding a hand under it will not have enough resistance and will be too easy).
~ Finally, you can use the help of someone who can see you on the mattress to make sure there are no obvious issues of alignment such as those that are in this diagram. If you stand up with “good posture” then your “helper” will be able to get a sense of your natural curves from the side and back and this can help them see if your posture is close to what it is when you are standing up and whether any part of your body is sinking in a little too far (usually the hips/ pelvis) or not enough (usually the upper body and shoulders). They can also make sure that your head and neck is also in good alignment when you are testing because this can affect how a mattress feels in the upper body area.
On your side … your spine should be relatively straight (like it is when you look at someone from behind) and your body profile along the side of your body should be similar to your standing position (shoulders and hips in roughly the same relative position). On your back … the spine and body profile should be similar to the side view when you are standing with no obvious areas where parts of you are sagging or sinking in too far or not enough (within reason).
If you test more objectively and specifically for alignment using these suggestions and the more subtle cues from your body in conjunction with some help then you should be much closer to your ideal than if you only test for more subjective ideas of “comfort”.
The testing guidelines in step 4 of the Mattress Shopping Tutorial should help.
Your alignment when you are lying down should be very similar to when you are standing up straight with good posture.
Phoenix