The growth of hair, regardless of sleeping position, is primarily influenced by genetics, health, and hair care practices rather than sleep position. On average, hair grows about half an inch (1.25 cm) per month, which translates to about 6 inches (15 cm) per year.
Sleeping upside down won't significantly alter the rate of hair growth. To promote healthy hair growth, focus on a balanced diet, proper hydration, and maintaining a good hair care routine. If you have specific concerns or questions about your hair health, it may be helpful to consult a dermatologist or a hair care specialist.