Patent Publication Number: US-10758058-B2

Title: Foam mattress with symmetrical wavy foam layers

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of, and claims priority under 35 U.S.C. § 120 from, U.S. patent application Ser. No. 16/436,375 entitled “Foam Mattress With Symmetrical Wavy Foam Layers,” filed on Jun. 10, 2019. application Ser. No. 16/436,375, in turn, is a continuation of U.S. patent application Ser. No. 13/780,100 entitled “Foam Mattress With Symmetrical Wavy Foam Layers,” filed on Feb. 28, 2013, now U.S. Pat. No. 10,357,115. The subject matter of each of the aforementioned patent documents is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to mattresses, and in particular to a zoned foam mattress. 
     BACKGROUND INFORMATION 
     A comfortable mattress is crucial to providing high quality sleep. One way of making a mattress more comfortable is to provide multiple lateral zones of differing firmness that correspond to different areas of the body of the user of the mattress. Different portions of the user&#39;s body exert different pressures on the mattress. Thus, the lateral zones under the user&#39;s shoulders and hips are made of softer foam than the lateral zones under the user&#39;s torso and legs in order to allow the user&#39;s shoulders and hips to sink into the mattress and to allow the user&#39;s spine to remain straight. 
     A typical zoned foam mattress is made by gluing together lateral rectangular blocks of foam in which adjacent blocks have differing hardnesses. The indentation load deflection (ILD) is one measure of hardness defined in the ISO 2439 standard. The standard defines ILD as the force that is required to compress the foam to a specified percentage of its original thickness using a circular plate of fifty square inches. For example, the 25%-compression ILD is the most commonly used ILD and is the number of pounds required to achieve the 25% compression. ILD is also measured at 40% and 60% compression. 
       FIG. 1  (prior art) is a cut-away perspective view of a conventional zoned foam mattress  10 . Mattress  10  includes an upper foam layer  11 , a zoned foam layer  12 , and a bottom foam layer  13 . Upper foam layer  11  is made of visco-elastic polyurethane foam, otherwise known as memory foam. A person using mattress  10  lies directly on upper layer  11  through a thin quilted fiber padding  14  sewn to the mattress cover  15 . Bottom foam layer  13  provides support for the other layers and is made of “high density” (HD) polyurethane foam. The term of art “high density” foam is somewhat of a misnomer because upper layer  11  of memory foam has a higher density than does the HD polyurethane foam. Typically, the HD foam used in mattresses has a density of between 1.5 to 2.5 pounds per cubic foot, whereas memory foam typically has a density between three and 5.5 pounds per cubic foot. Zoned foam layer  12  rests on bottom foam layer  13 . 
     Zoned foam mattress  10  is a Queen size mattress that is sixty inches wide and eighty inches from the top end  16  to the bottom end  17  of mattress  10 . Zoned foam layer  12  includes longitudinally spaced, transversely extending lateral regions  18 - 21  of foam. A first lateral region  18  is located at top end  16  of mattress  10 . A second lateral region  19  is disposed between first lateral region  18  and a third lateral region  20 . The user of mattress  10  sleeps with his or her shoulders over second lateral region  19  and his or her hips above a fourth lateral region  21 . Regions  19  and  21  have a lower ILD than do regions  18  and  20 . Consequently, the user&#39;s shoulders and hips sink deeper into regions  19  and  21 . 
     However, forming a mattress by gluing together lateral blocks of foam having different degrees of hardness complicates the manufacturing process and adds to the cost of the mattress. In addition, the many glued joints create more places for the mattress to come apart. A method is sought for making a zoned foam mattress that does not require gluing together lateral rectangular blocks of foam to form the zones of different hardness. 
     SUMMARY 
     A symmetrical zoned foam mattress includes an upper foam layer with a wavy lower side and a lower foam layer with a wavy upper side. The foam of the upper layer is softer than the foam of the lower layer. For example, the upper layer is made of softer memory foam, and the lower layer is made of harder high density foam (HD foam). The lower foam layer has a top, middle and bottom lateral valleys. The wavy lower side of the upper foam layer is glued to the wavy upper side of the lower foam layer. The upper foam layer together with the lower foam layer have a combined thickness that remains constant from the top head side to the bottom foot side of the mattress. The top lateral valley has a minimum located within eighteen inches of the top head end of the lower foam layer. The bottom lateral valley has a minimum located within eighteen inches of the bottom foot end of the lower foam layer. A center plane intersects the lower foam layer at a minimum of the middle lateral valley. The minimum of each of the top and bottom lateral valleys is about fifteen inches from the end of the mattress regardless of whether the consumer chooses to use the top or bottom end as the head of the mattress. 
     A cross section of the wavy upper side of the lower foam layer forms a curve that is a mirror image of itself on either side of a center plane through the lower foam layer. Thus, the top lateral valley and the bottom lateral valley are disposed symmetrically to the center plane. Between the top lateral valley and the middle lateral valley, the lower foam layer has a top lateral hill with a maximum located within eighteen inches of the center plane. The zoned foam mattress has a lower indentation load deflection (ILD) above the top lateral valley than above other regions immediately adjacent to the top lateral valley. Thus, the memory foam of the top layer above the top lateral valley of the lower layer imparts an ILD to the mattress above the top lateral valley that allows a person&#39;s shoulders to sink into the mattress so as to keep the person&#39;s spine straight. 
     A method of making a symmetrical zoned mattress with wavy upper and lower foam layers includes cutting first and second slabs along a predetermined curve. The first slab is made of harder foam than the second slab. For example, the first slab is made of HD foam, and the second slab is made of memory foam. The first slab of foam is cut to form a first top lateral valley, a first top lateral hill and a middle lateral valley in an upper side of a first foam layer. Cutting the first slab forms a top piece and a bottom piece. The bottom piece includes the first top lateral valley and the first top lateral hill. The top piece has a first bottom lateral valley. 
     The first foam layer is formed by attaching the bottom piece to the top piece at the minimum of the middle lateral valley. The first top lateral valley and the first bottom lateral valley are disposed symmetrically to a center plane of the first foam layer that intersects the upper side of the first foam layer at the minimum of the middle lateral valley. The minimum of the first top lateral valley is located within eighteen inches of the top head side of the first foam layer. In addition, the second slab of foam is cut to form a second top lateral hill, a second top lateral valley and a middle lateral hill in a lower side of a second foam layer. 
     The lower side of the second foam layer is then placed over the upper side of the first foam layer such that the second top lateral hill fits into the first top lateral valley, and the middle lateral hill fits into the middle lateral valley. The first foam layer together with the second foam layer have a combined thickness that is constant. The first top lateral valley has a minimum located within eighteen inches from a maximum of the first top lateral hill. An indentation load deflection (ILD) above the top lateral valley is lower than above other lateral regions immediately adjacent to the top lateral valley after the lower side of the second foam layer is placed over the upper side of the first foam layer. 
     Further details and embodiments are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate embodiments of the invention. 
         FIG. 1  (prior art) is a perspective view of a zoned foam mattress with lateral rectangular blocks of foam exhibiting differing hardnesses. 
         FIG. 2  is a perspective view of a zoned foam mattress showing the cross sectional curve of the wavy upper surface of a lower layer. 
         FIG. 3  shows a zoned foam mattress with a symmetrical curve between the wavy lower side of an upper foam layer and the wavy upper side of a lower foam layer. 
         FIG. 4  is a perspective diagram of the zoned foam mattress of  FIG. 3  showing the valleys and hills between the upper foam layer and the lower foam layer. 
         FIG. 5  is a cut-away perspective view of another embodiment of a zoned foam mattress with a symmetrical curve between upper and lower foam layers. 
         FIG. 6  is a flowchart of steps for manufacturing a symmetrical zoned foam mattress with wavy layers that does not involve gluing together lateral foam blocks of different hardness. 
         FIG. 7  illustrates how the steps of the method of  FIG. 6  are performed on slabs of memory foam and HD foam to manufacture a symmetrical zoned foam mattress. 
         FIG. 8  illustrates an adaptation of the method of  FIG. 6  that allows the symmetrical zoned foam mattress to be manufactured more efficiently using long sheets of foam. 
         FIG. 9  is a cross-sectional view showing a user&#39;s shoulders sinking into the thicker memory foam over the second top lateral hill, and the user&#39;s hips sinking into the thicker memory foam above the middle lateral hill of the upper wavy foam layer. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a perspective view of a zoned foam mattress  22  that does not include lateral, rectangular blocks of foam. Instead, the zones of varying hardness are formed by adjusting the relative thicknesses of an upper layer  23  of softer foam compared to a lower layer  24  of harder foam. The cross sectional curve of the upper surface of lower layer  24  corresponds to the contour of the body of a user  25  lying on mattress  22 . The softer upper layer  23  is thicker under the user&#39;s shoulders and hips than under the user&#39;s torso and head. The curve between the wavy lower side of upper layer  23  and the wavy upper side of lower layer  24  is custom fit to the body of user  25 . 
     One method of making zoned foam mattress  22  involves forming lower layer  24  in a mold whose lid produces the shape of the wavy upper side of lower layer  24 . The lid is then removed from the cured lower layer  24 , and a softer foam is poured over lower layer  24  to form upper layer  23  with a planar upper surface. But molding individual foam layers of mattress  22  is time consuming and therefore costly. A mattress with wavy foam layers is sought that can be manufactured in a more efficient manner. 
       FIG. 3  shows a pressure-relief, zoned foam mattress  30  of a first embodiment of the present invention that includes an upper foam layer  31  with a wavy lower side  33  and a lower foam layer  32  with a wavy upper side  34 . Zoned foam mattress  30  can be manufactured without molding the wavy sides of layers  31 - 32  and without gluing together lateral foam blocks to form zones of different hardness. The zones of differing hardness of mattress  30  correspond to the thicker and thinner areas of the softer foam of upper foam layer  31 . Upper foam layer  31  is made of visco-elastic polyurethane foam (memory foam), whereas lower foam layer  32  is made of high-density polyurethane foam (HD foam). Although HD foam typically has a lower density than memory foam, the HD foam used in mattress  30  is harder than the memory foam used in mattress  30 . Harder foam has a higher indentation load deflection (ILD). 
     In another embodiment, lower foam layer  32  is formed from natural latex rubber (as opposed to HD foam) that also is harder than the memory foam used in the mattress. Wavy lower side  33  of upper foam layer  31  is adjacent to wavy upper side  34  of lower foam layer  32 . Mattress  30  has a top portion  35 , a middle portion  36  and a bottom portion  37 . In  FIG. 3 , mattress  30  is an 80-inch long queen size mattress in which top portion  35  is thirty inches long, middle portion  36  is twenty inches long, and bottom portion  37  is thirty inches long. In other embodiments, mattress  30  is an 80-inch long king size mattress or a 75-inch long twin size mattress.  FIG. 3  shows the head  38  of user  25  resting towards the top  39  of mattress  30 , and the feet of user  25  are near the bottom  40  of mattress  30 . However, the varying thicknesses of the foam layers  31 - 32  are symmetrical with respect to a center plane  41 , and the head  38  of user  25  could just as comfortably be positioned toward the bottom  40  of mattress  30 . It is also less expensive to produce mattress  30  because the cross-sectional curve  42  between foam layers  31 - 32  is symmetrical with respect to center plane  41 . 
       FIG. 4  is another perspective view of zoned foam mattress  30  showing the valleys and hills between upper foam layer  31  and lower foam layer  32  without user  25  lying on the mattress. Lower foam layer  32  has a first top lateral valley  43 , a middle lateral valley  44  and a bottom lateral valley  45 . First top lateral valley  43  has a minimum  46  located within eighteen inches of the top  39  of lower foam layer  32 , and bottom lateral valley  45  has a minimum  47  located within eighteen inches of the bottom  40  of lower foam layer  32 . Lower foam layer  32  also has a first top lateral hill  48  between first top lateral valley  43  and middle lateral valley  44 . First top lateral hill  48  has a maximum  49  located within eighteen inches of center plane  41 . Lower side  33  of upper foam layer  31  is positioned over upper side  34  of upper foam layer  31  such that a second top lateral hill  50  of upper foam layer  31  fits into first top lateral valley  43  of lower foam layer  32 , and a middle lateral hill  51  of upper foam layer  31  fits into middle lateral valley  44  of lower foam layer  32 . Second top lateral valley  52  of upper foam layer  31  fits over first top lateral hill  48  of lower foam layer  32 . The minimum  46  of first top lateral valley  43  is located within eighteen inches of maximum  49  of first top lateral hill  48 . 
     Center plane  41  intersects lower foam layer  32  at a minimum  53  of middle lateral valley  44 . The cross section of wavy upper side  34  follows curve  42 , as does the cross section of wavy lower side  33 . Upper foam layer  31  together with lower foam layer  32  have a combined thickness that remains constant from the top  39  to the bottom  40  of mattress  39 . The portion of curve  42  extending from center plane  41  towards the top  39  of mattress  30  is a mirror image of the portion of curve  42  extending from center plane  41  towards the bottom  40  of mattress  30 . Thus, top lateral valley  43  and bottom lateral valley  45  are disposed symmetrically relative to center plane  41 . The memory foam above first top lateral valley  43  of lower foam layer  32  imparts an indentation load deflection (ILD) to the lateral region above top lateral valley  43  that allows the shoulders of user  25  to sink into mattress  30  so as to keep the user&#39;s spine straight. Thus, mattress  30  has a lower ILD above top lateral valley  43  than above other regions immediately adjacent to top lateral valley  43 . Similarly, the hips of user  25  resting on the greater thickness of softer foam at middle lateral valley  44  sink into mattress  30  more than does the user&#39;s torso. The user&#39;s spine can remain straighter if both the user&#39;s hips and shoulders sink farther into the mattress than do the user&#39;s torso and legs. 
       FIG. 5  is a cut-away perspective view of another embodiment of symmetrical zoned foam mattress  30 . Mattress  30  includes a top foam layer  55 , upper foam layer  31 , lower foam layer  32 , and a bottom foam layer  56 . Bottom layer  56  provides support for the other layers and is made of a harder HD foam than is lower foam layer  32 . Both top foam layer  55  and upper foam layer  31  are made of memory foam (visco-elastic polyurethane foam) that contains green tea and is colored green. User  25  lies directly on layer  55  through a thin quilted fiber padding  57  of the mattress cover. The green tea in top layer  55  acts as an antiodorant such that less of the chemical smell of the memory foam is perceived by user  25 . In addition, people tend to perspire more while sleeping on memory foam. The bacteria and mold that would otherwise develop in the moist environment of the memory foam are killed by the green tea additive to the foam. 
     Mattress  30  is configured to provide optimum support for the largest percentage of North American consumers. The region  58  of the first top lateral valley  43  is about ten inches wide. In addition, region  58  of valley  43  is about ten inches from the top  39  of mattress  30 . There are also about ten inches between region  58  of valley  43  and the region of middle lateral valley  44 . The average consumer, regardless of body height, sleeps with his or her head the same distance from the top of the mattress. Thus, the average North American consumer sleeps with his or her shoulders about fifteen inches from the top of the mattress. The middles of lateral valleys  43  and  45  are both about fifteen inches from the “head” of mattress  20  regardless of whether user  25  chooses to use the top  39  or the bottom  40  as the head of the mattress. By placing two symmetrical lateral valleys  43  and  45  at the top  39  and bottom  40  of mattress  30 , user  25  cannot lay the mattress down on a bed frame in the incorrect orientation with the head of the mattress towards the foot of the bed frame. Regardless of how the mattress is laid down on the bed, the valleys of softer foam for the shoulders are present without fail within the correct area of the mattress to contact the user&#39;s shoulders. The region of middle lateral valley  44  occupies the entire length of mattress  30  from thirty inches from the top  39  to thirty inches from the bottom  40  of the mattress. Thus, for a 75-inch long twin size mattress, the region of valley  44  is about fifteen inches long. For an 80-inch long queen size mattress, the region of valley  44  is about twenty inches long. 
     Mattress  30  does not have a zone of the wavy foam layers that is specifically tailored to the legs of a person reclining on the mattress. Instead, a user&#39;s legs lie over the regions of the lateral valleys  43  or  45  positioned for the shoulders. The benefit of always positioning a user&#39;s shoulders correctly over the region of a lateral valley, regardless of whether the user lies toward the top  39  or bottom  40  of mattress  30 , outweighs the lack of optimum leg support. Providing a foam zone with an indentation load deflection (ILD) specifically suited to support a user&#39;s legs contributes much less to keeping the reclining user&#39;s spine straight than does positioning lateral regions with the appropriate ILDs beneath the user&#39;s shoulders and hips. Moreover, foam zones intended to support the legs are often ineffective. Where a tall man and a short woman are lying on the same mattress, their shoulders will likely rest at the same distance from the end of the mattress, whereas their legs will likely not rest in the same lateral region. Thus, any foam zone with an ILD specifically suited to support a user&#39;s legs would not be in the appropriate position for both the tall man and the short woman. Instead of offering multiple ineffective indentation zones, mattress  30  provides a shoulder foam zone that is always correctly positioned and a variable width hip foam zone that is appropriate for the largest percentage of North American consumers. 
       FIG. 6  is a flowchart illustrating steps  61 - 65  of a method  60  of manufacturing symmetrical zoned foam mattress  30 . Method  60  does not involve gluing together lateral foam blocks to form zones of different hardness. Moreover, method  60  forms the wavy sides of layers  31 - 32  without molding foam to form the waves. The steps of method  60  are illustrated in  FIG. 7 . The wavy foam layers  31 - 32  are formed from a first slab  68  of harder foam and a second slab  69  of softer foam. For example, first slab  68  is used to make lower foam layer  32  and has a density of about three pounds per square foot and an ILD of about fifteen. Second slab  69  is used to make upper foam layer  31  and has a density of about four pounds per square foot and an ILD of about nine. Thus, the visco-elastic polyurethane foam (memory foam) of first slab  68  has a higher density than the high-density polyurethane foam (HD foam) of second slab  69 . Bottom foam layer  56  has a density of about three pounds per square foot and an ILD of about twenty five. Thin top foam layer  55  has a density of about 1.5 lbs/sqft and an ILD of about five. Each of the slabs of foam  68 - 69  is half as long as the mattress  30 . To make an 80-inch-long queen size mattress, the slabs  68 - 69  are each forty inches long. 
     In a first step  61 , first slab  68  is cut to form first top lateral valley  43 , first top lateral hill  48  and middle lateral valley  44  in wavy upper side  34 . The cut  70  in first slab  68  forms a bottom piece  71  and a top piece  72 . First top lateral valley  43  and first top lateral hill  48  are on bottom piece  71 . Top piece  72  has a first bottom lateral valley  73 . 
     In step  62 , lower foam layer  32  is formed by attaching bottom piece  71  to top piece  72  at minimum  53  of middle lateral valley  44  as shown in step (e) of  FIG. 7 . Top piece  72  is flipped upside down before it is attached to bottom piece  71 . Because the same cut  70  forms both bottom piece  71  and top piece  72 , the curve of upper side  34  is symmetrical on either side of middle lateral valley  44  at which bottom and top pieces  71 - 72  are attached. Thus, first top lateral valley  43  and first bottom lateral valley  73  are disposed symmetrically to a center plane  74  of lower foam layer  32  that intersects the upper side  34  of lower foam layer  32  at minimum  53  of middle lateral valley  44 . 
     In step  63 , second slab  69  is cut to form second top lateral hill  50 , second top lateral valley  52  and middle lateral hill  51  in wavy lower side  33  of upper foam layer  31 . The cut  75  in second slab  69  forms a top piece  76  and a bottom piece  77 . Cut  75  has the same shape as cut  70 . Second top lateral hill  50  and second top lateral valley  52  are on top piece  76 . Bottom piece  77  has a second bottom lateral hill  78 . 
     In step  64 , upper foam layer  31  is formed by attaching top piece  76  to bottom piece  77  at a maximum  79  of middle lateral hill  51  as shown in step (e) of  FIG. 7 . Bottom piece  77  is flipped upside down before it is attached to top piece  76 . Because the same cut  75  forms both top piece  76  and bottom piece  77 , the curve of lower side  33  is symmetrical on either side of middle lateral hill  51  at which top and bottom pieces  76 - 77  are attached. Thus, second top lateral hill  50  and second bottom lateral hill  78  are disposed symmetrically to center plane  74  of upper foam layer  31  that intersects upper side  33  of upper foam layer  31  at maximum  79  of middle lateral hill  51 . Top and bottom pieces  76 - 77  are attached by gluing  80 , as are top and bottom pieces  72 - 71 . 
     In step  65 , lower side  33  of upper foam layer  31  is placed over upper side  34  of lower foam layer  32  such that second top lateral hill  50  fits into first top lateral valley  43 , and middle lateral hill  51  fits into middle lateral valley  44 . First top lateral valley  43  has a minimum  46  located within eighteen inches from maximum  49  of first top lateral hill  48 . Upper foam layer  31  is attached to lower foam layer  32  by gluing  81 . After upper foam layer  31  is attached to lower foam layer  32 , upper foam layer  31  together with lower foam layer  32  have a combined thickness that is constant, as shown in step (f) of  FIG. 7 . 
       FIG. 8  illustrates an adaptation of method  60  that allows the symmetrical zoned foam mattress  30  to be manufactured even more efficiently. Instead of beginning with short slabs of foam, as shown in  FIG. 7 , mattress  30  is made with long sheets of foam. In one embodiment, the long sheets are made in a continuous process. As each long foam sheet moves along a production line, a cutting device moves up and down to create a continuous cut  82  that repeats the curve  42 . To make an 80-inch-long queen size mattress, the curve  42  is repeated every eighty inches. 
     Two long foam sheets are used: one made of HD foam  83  and the other made of memory foam  84 . The HD foam  83  is used to make lower foam layer  32 , and the memory foam  84  is used to make upper foam layer  31 . The foam sheets need not have the same thickness as shown in  FIG. 8 . A thicker sheet of HD foam  84  is usually used because HD foam is less expensive than memory foam. The lower foam layer  32  of HD foam acts as a support for the softer upper foam layer  31 . Despite the different thicknesses of the HD and memory foam, the same cut  82  with the same curve  42  must be applied to both sheets of foam. Cut  82  is made in the middle of the thickness of the foam sheets. 
     The two long foam sheets of HD foam  83  and memory foam  84  are cut into four wavy layers, designated as  85 ,  86 ,  87 ,  88  in  FIG. 8 . Wavy layer  85  of HD foam  83  is flipped over such that its flat side is down. Wavy layer  88  of memory foam  84  is flipped over such that its flat side is up. In a first embodiment of the method, upper wavy layers  87 - 88  are shifted 180 degrees along curve  42  (half of the curve), placed over lower wavy layers  86 - 85 , respectively, and then cut every eighty inches. 
     In a second embodiment, the wavy layers  85 - 88  are first cut and then placed over each other. Lower wavy foam layer  85  is cut to form a first planar top side  89  that is approximately forty inches from middle lateral valley  44 . Lower wavy foam layer  86  is cut in the same manner. Thus, middle lateral valley  44  is in the middle of each 80-inch segment of foam layers  85 - 86 , as shown in  FIG. 8 . Upper wavy foam layer  87  is cut to form a second planar top side  90  that is approximately forty inches from middle lateral hill  51 . Upper wavy foam layer  88  is cut in the same manner. Lower side  33  of upper layer  87  is then place over upper side  34  of lower layer  86  such that middle lateral hill  51  fits into middle lateral valley  44 . Similarly, upper layer  88  is place over lower layer  85  such that middle lateral hill  51  fits into middle lateral valley  44 . After the placing of lower side  33  of upper foam layer  87  over upper side  34  of lower foam layer  86 , first planar top side  89  and second planar top side  90  are coplanar and form the top  39  of mattress  30 . 
     Cutting lower wavy foam layer  85  to form first planar top side  89  also forms a first planar bottom side  91  of the adjacent 80-inch segment to the right in  FIG. 8 . First planar bottom side  91  is approximately forty inches from the middle lateral valley  44  of the adjacent segment to the right. As shown in  FIG. 8 , for each segment of lower foam layer  85 , upper side  34  from the minimum  53  of middle lateral valley  44  towards first planar top side  89  on the right of the segment is a mirror image of upper side  34  from minimum  53  of middle lateral valley  44  towards first planar bottom side  91  on the left of the segment. 
     By cutting the sheets of foam along curve  42 , each segment and also each mattress has lateral wavy foam zones that are symmetrical from top to bottom. This prevents the user  25  from placing the head side of the mattress at the foot of the bed, which would be possible if the curve of the wavy foam zones was not symmetrical with respect to the middle of the mattress. Cutting the sheets of foam along curve  42  also provides wavy foam zones in the correct position for the body zones of the largest segment of North American users. Regardless of body height, the average North American consumer sleeps with his or her shoulders about fifteen inches from the top of the mattress, which falls at the maximum of second top lateral hill  50  of the upper layer  31  of memory foam. 
       FIG. 9  illustrates how mattress  30  permits the user&#39;s spine  92  to remain straight when the shoulders  93  and hips  94  can sink farther into the mattress.  FIG. 9  shows that the user&#39;s shoulders  93  sink into the thicker memory foam over second top lateral hill  50 , and the user&#39;s hips  94  sink into the thicker memory foam above middle lateral hill  51 . In order to achieve spinal alignment, the supporting forces of the mattress, under the load of the reclining body, must vary along the body to match the body density and shape. But in order for the mattress to be comfortable, the supporting pressures of the mattress against the skin must also be even over the entire body. A straight side-lying spinal alignment of a reclining user is generally considered to be that alignment in which the spine is straight and on the same center line as the legs and the head as shown by the dots along spine  92  of user  25  in  FIG. 9 . 
     Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Although a particular curve  42  is cut into the long sheets of foam to make mattress  30  as shown in  FIG. 8 , other shaped curves can also be used. Any other curve can be used that is both symmetrically relative to center plane  41  and that conforms to the locations of the shoulders and hips of a selected group of users. The top and bottom of each segment of the lower wavy foam layers  85 - 86  is then cut so as to be offset by 180 degrees of the curve from the top and bottom cuts in the upper wavy foam layers  87 - 88 . Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.