Because your spine is
"S" shaped, flat beds create gaps where your spine is not supported.
It's a physiological fact that muscles in use
are in a state of tension. When you are forced
to hold your head up, stretch your legs and spine,
support your arms and legs, and move your body
yourself to change your position (or improvise
contoured support with pillows as you must do
in a flat bed), your muscles stay in tension.
That is why when you lie on your back in a flat
bed, you have to constantly move your body to
Even turning onto your side doesn't help for
long, because lying on your side eventually cuts
off blood circulation, hampers breathing, and
gravity exerts disturbing downward pressure on
your internal organs. Thus you turn again, because
the flat surface you are sleeping on is actually
causing your curved body pain and forcing your
muscles to stay in tension.
When asleep, you unconsciously move into one
position after another to escape this pain and
tension, but since your bed is flat and can't
conform to your body's curves, you toss and turn
all night long. A good night's sleep is difficult
to achieve on a flat bed!