It's just gone 3am.

The piercing sound of an alarm clock rouses Lucy Yeomans from her slumber.

While the rest of the world sleeps, Ms Yeomans sets off to begin her day under a dark sky.

A lifelong early riser, Ms Yeomans, 29, has found the perfect job for her early-morning self as a track rider for leading Caulfield horse trainer, Ciaron Maher.

For more than four years, Ms Yeomans has had a strict sleep regime of going to bed at 8.30pm for a 3.50am start at the track.

"You couldn't do my job if you didn't love it, because of the crazy hours," she said. "The roads are empty, but when you get to the track everybody is up and about."

According to new research, Ms Yeomans' natural affection for early mornings can be traced back to evolution.

A healthy mixture of early birds, night owls and wakeful elderly people may have evolved as a clever way of protecting the community from wild beasts and dangerous environments during the night, a US study has found.

By tracking the sleep activity of 22 modern-day Hadza hunter-gatherers in Tanzania, researchers from the Royal Society found over a period of 20 days that there were only 18 minutes when no one was awake.

The scientists believe the strategy explains why there remains a wide variety in the sleeping habits of modern-day humans, including elderly people more likely to be awake than youngsters.

While sleep is essential for human survival, for our Paleolithic ancestors it also represented a time of extreme vulnerability.

The researchers proposed that group-living animals shared the task of vigilance during rest periods.

In other words, somebody must always be awake to protect the tribe from dangers such as the jaws, horns or claws of wild beasts.

One of the scientists leading the study and a senior researcher at Duke University, Dr David Samson, says the variation in sleep patterns and higher rates of wakeful older individuals in modern humans may be a legacy of natural selection acting to reduce the dangers of sleep.

"We show that asynchronous (out of synch) sleep is common, with one or more individuals examined awake during 99.8 per cent of the rest period," Dr Samson said.

"We also found that variation in chronotype (propensity for an individual to sleep at a particular time during a 24-hour period) facilitates this effect, and is itself influenced by age."

A median of eight individuals were awake throughout the night.

Dr Samson said the results showed one or more individuals were awake during 99.8 per cent of sampled time periods between when the first person went to sleep and when the last person awoke.

The researchers say in a group situation, vigilance can be achieved through behaviour such as variation in sleep timing, periodic awakenings, and periods of time spent in lighter stages of sleep, to reduce the risk of danger.

Although there are no birds chirping or, hopefully, wild beasts lurking when Ms Yeomans rises each morning, she agrees with the concept of protecting the tribe.

"It seems a logical way for people or animals to survive in dangerous environments," she said.

"With wild animals in the wilderness there is always somebody watching out over the herd, so the idea makes sense to me.

"There has never been a real reason why I have always woken early in the morning; I have always considered it just to be the way I am."

Despite averaging fewer hours sleep than she'd often like, Ms Yeomans said nothing compared to the quiet stillness of an early morning on a cold winter's day.

"I reckon it's the best part of the day. You get to see the sunrise every morning too, which is pretty special."