The most important sleep disturbing factor is very quiet, it's your bed. It is impossible to sleep upright, as you would collapse as soon as your muscles start to relax. Sleeping in a seated position is feasible for one or two cycles, but then the pressure on your backside increases too much.
So, lying is the best and universal sleep position. In this position, the contact plane with the support is largest and your muscles can relax best.
Beds and mattresses are available in all kinds of models and materials. But however hard or soft they are, you always register the pressure of your body on a mattress. The larger the contact plane between the body and the mattress, the smaller the counter-pressure per square centimeter of skin surface. After you have been in bed for some time, you will automatically take off the pressure from the pressed parts of your body by turning over.
The pressure on your body depends on your dominant sleep position, your weight, the size of the contact plane, and the hardness of your mattress.
As long as you are in perfect health, you will easily try to find a larger contact plane with your mattress and external stimuli will not wake you up as easily.
But… what happens if something is ailing you?
A lot of reports treat the subject of sleeping in a condition of weightlessness. Nevertheless, to my knowledge, there are no findings based on a polyhypnogram and therefore we do not know whether sleeping in a condition of weightlessness approaches perfection.
What we do know is that the contact plane between the body and the mattress must increase as the body becomes more susceptible to pressure; and therefore the mattress has to be more elastic.
If the body registers too much counter-pressure during sleep, you turn over to avoid pressure damage. Turning over requires an effort from the structural muscles. A lot of people already turn over after 10 to 20 minutes when lying in their dominant sleep position on a hard surface. Turning over in the deep sleep and dream sleep stages interrupts these stages and the time that is needed to regain muscle relaxation limits and impairs the quality of your sleep. Even on an extremely elastic bed, you continue to register pressure and to turn over, however less frequently. The strongest disturbing factor however, is a hard mattress. Sleeping on a hard bed not only shortens your deep sleep and REM sleep, but also causes morning stiffness, shoulder and hip pains, which in turn are inductive to neck pain, low back pains or headache.
After a few hours of sleep, your body already is tired of your bed.
If you get up in the morning and still feel tired, there may be something wrong with your bed although you never had any complaints before.