There are many jokes that center around older adults waking up before the sun, and even more about teenagers’ late-sleeping habits. Turns out there’s truth to them: The time our body naturally goes to sleep and wakes up is not only part of our genetics, but part of the natural aging process, too.

As we age, our bodies change both internally and externally, which is a major factor behind the sleep changes that come later in life. “Like most of the things that change with age, there’s not just one reason, and they are all interconnected,” said Cindy Lustig, a professor of psychology at the University of Michigan.

We asked Lustig and other experts to break down the main reasons why this occurs, and what you can do to push back if you just want those few extra hours of Zzzs.

Read the complete article in HuffPost