You might be surprised to find out when this sleep deprivation begins. Walker says that individuals shouldn’t think that this only occurs once they reach their senior years. Sleep decline often begins in the late 20s and early 30s, and continues on a steady downhill track as time goes on. In fact, by the time a person hits 50, they will only have about 50 percent of the deep sleep that they were getting in their early 20s. By 70, individuals have little, if any, high-quality deep sleep. Instead of going through the proper sleep cycles that ensure a well-rested night’s sleep, they wake up throughout the night, constantly inhibiting the deep sleep that’s essential to proper brain function. “It’s probably one of the most dramatic physiological changes that happen with aging,” Walker says.