Peoples sleep duration on average gradually and steadily declines from as much as 11-19 hours as an infant, down to 5-8 on average for 65+. There is no rapid su

Peoples sleep duration on average gradually and steadily declines from as much as 11-19 hours as an infant, down to 5-8 on average for 65+. There is no rapid sudden drop, and possibly no age where it stops going down either. However, peoples individual need for sleep varies substantially on top of that, and can go up or down based on a lot of other factors. Medications, mental health, sleep apnea, diet, exercise, etc. can influence your need for sleep and how restful the sleep you get is. If you're sleeping 12 hours and still feeling awful, there is likely something very wrong you need to look into. I'd go to a doctor and get a sleep study, but if nothing else you can get a logging pulse oximeter, and/or sleep tracker like an Oura ring. It is possible you are not really sleeping but having short waking events, and/or apnea events from sleep apnea that is keeping your sleep from being restful.