How can God Almighty's saying: "They used to sleep but little of the night" be reconciled with doctors' recommendations of the necessity of sleeping for 8 or 9 hours for the body's stability, and what are the best ways to apply this?

Allah knows best what benefits His servants, and His truthful words do not contradict established medicine. Sleeping at night is not obligatory, as the siesta (daytime nap) aids in the night vigil (qiyam al-layl), and sufficient sleep both night and day (siesta) is beneficial for the body and the heart. This combines the right of Allah with the right of the body. Excessive sleep is harmful to the heart and the body.