FOR SIDE SLEEPERS
Majority of people sleep in this fetal position. This is a great position for mums-to-be and those frequently suffering from heartburn, acid reflux based on the body’s pressure points.
A medium to medium soft mattress will best keep your spine aligned while its cushioning comfortably contours to your body’s natural curve
 
FOR BACK SLEEPERS
If you mostly sleep on your back, a firm or orthopedic mattress best gives you a good night’s sleep. To maintain neutral spine alignment, support your back and other body pressure points as well as prevent your torso from sinking in, a firm, orthopedic mattress is recommended.  
 
FOR FRONT SLEEPERS
In case you sleep on your stomach, you may be prone misaligning the natural curve of your spine and possibly straining your neck and shoulders.

A firm to medium firm mattress will best support the natural alignment of your spine. Too soft a mattress may cause you to sink in, arch your spine whereas too firm a mattress may cause discomfort at your pressure points e.g. hips, knees, shoulders and neck.