The risk of suffocation during sleep because of bedding, sheets or covers is something we worry about in newborns  and infants whose central nervous system, including the brain stem (responsible for controlling activities such as respiration and heart beat) are still immature or developing; this is not the case in normal adults. And of course the act of breathing is heavily involuntary (that's why people cannot end their lives but stopping their breathing), and should anything interrupt breathing, there is a reflexive response that triggers the person to wake up and gasp for air. Along the same line, the more important factor is the positioning of the head and back during sleep - they ought to be aligned to allow for comfortable breathing.