At 5 years after a BBL, sleeping on your back should not meaningfully change the surgical result. The transferred fat that survived the early healing period is generally stable, and normal sleeping pressure years later should not make the result disappear or shift. Waking with numbness in the legs or buttocks is more likely related to temporary pressure on nerves or circulation from position, mattress firmness, or a spine/nerve issue rather than the BBL itself. Try changing positions, using pillows to reduce pressure, and avoiding long periods in one position. If the numbness is frequent, lasts after you get up, or concerns you, it is worth discussing with your treating surgeon or primary physician. Seek prompt evaluation if it is one-sided, comes with weakness, significant pain, swelling, color change, or bowel/bladder symptoms.