As people age, their bodies alter both internally and externally as their brain and nervous system go through natural changes.

This is a major factor behind the sleep changes that come later in life, and yes, many people reaching retirement tend to wake up earlier.

“The wiring of the brain is likely not sensing … and responding to the inputs as well as it should because it’s an ageing brain,” Dr Sairam Parthasarathy, the director of the Center for Sleep and Circadian Sciences at the University of Arizona Health Sciences, told Huffington Post.

These inputs include sunset, sunlight, meals, social cues and physical activity that help the brain sense where it is in the 24-hour circadian cycle.

“These are all what we call time givers, or they give time to the brain,” he said.

For example, after a younger person has had dinner, the brain understands that bedtime is in a few hours. For someone older, this connection may not happen.

The nerves that are supposed to give the brain time cues have undergone the same amount of degeneration as the brain, Dr Parthasarathy said. This inability to sense time cues is part of the reason why older people tend to get tired before their children or grandchildren. And, as a result, wake up fully rested and earlier than the rest of the world.

The light a person’s eyes take in is part of the reason, too.

A person’s vision may change as they age which lessens the intensity of the degree of light stimulation that their brain receives, which plays an important role in setting the circadian clock and keeping it on track.

If there is less light is getting into the eyes, the body starts to release melatonin (the sleep hormone) earlier than it should. For younger people, melatonin “starts rising after sunset,” Dr Parthasarathy said, which is why you generally feel tired a few hours after. For people whose brain thinks sunset was earlier, their perceived sunset is earlier, which makes them tired sooner in the evening. And going to bed sooner means waking up earlier.

To try and improve sleep, Dr Parthasarathy urged people to expose themselves to bright light in the late evening by going for a walk outside before the sun sets, reading a book on a bright laptop, leaving lights on in the person’s home or watching TV on a bright screen.

The bright lights will tell the brain that the sun hasn’t set yet, which will hold the melatonin production, he said. To help yourself stay up a little later (and sleep a little later as a result), Dr Parthasarathy said you should try these things 30 to 60 minutes before sunset, which will vary depending on the time of year and where you live.

Medical advice is to avoid the intake of alcohol before going to bed. It might make you go to sleep more easily but it will disrupt the quality of sleep.

However, changes in sleep patterns are a part of life. Some of these factors are out of a person’s control, but you can counteract them with healthy habits to get a better rest.