There is no scientific evidence to suggest that sleeping in a certain position can prevent breasts from sagging. Breast sagging, also known as ptosis, is a natural part of the aging process and can be influenced by factors such as genetics, weight fluctuations, and pregnancy.
However, there are some measures you can take to keep your breasts healthy and potentially reduce the risk of sagging, such as wearing a properly fitted bra during the day and engaging in regular exercise to strengthen the chest muscles. Maintaining a healthy weight and avoiding smoking may also help prevent breast sagging.
It's important to remember that breast shape and size can vary greatly among individuals, and some degree of sagging may be inevitable regardless of lifestyle factors. If you have concerns about the appearance of your breasts, you may wish to consult with a plastic surgeon or other healthcare professional for advice.