& Nbsp; & nbsp; highly functional sofa bed sitting surface is preferably between 35-42 cm , which is about equal to the height of one leg . Too high or too low is not conducive to the size of human muscle relaxation , prolonged use is likely to cause physical pain. Seat depth between 48-55 cm more appropriate, that is, when affixed to the back as far as possible against the back of the sofa , the person's knees should still outside the seat surface . Counting from the ground , the height of the backrest of a sofa bed should be maintained at 68-72 cm , in this context , the human body is the most comfort . In addition, the inclination of the backrest is preferably between 100-108 degrees, and the height of the armrest between 62-65 cm more appropriate. & Nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp;