For people in their twenties, about half of their sleep time each night is spent in which stage

Young adults (late teens, early 20s) have the lowest rate of sleep disturbances of any age group with the possible exception of babies. They are past the common childhood disorders (night terrors, sleepwalking) but do not yet suffer the insomnia of middle age or the fragmented sleep of old age. This is a generalization, of course, but we can say that young adulthood is the golden age of sleep.

It's called the nephron.