There are a number of factors that change how and when we sleep, from shifts in brain activity to a loss of brain cells that tell us when it’s time to rest, reports Business Insider.

A typical 25-year-old plunges half a dozen times into several hours of sustained deep sleep throughout the night. In contrast, the average 70-year-old brain shuffles quickly in and out of moderate-level sleep, spending only a few minutes in the deepest phase of rest and far more time in shallow sleep or complete wakefulness. The transition between being asleep and awake also becomes far more abrupt as we age. This is probably why older people are generally more likely to call themselves “light sleepers.”