President Donald J. Trump's physician said on Tuesday that the president only sleeps about four or five hours a night.

That's a lot less than the average seven or eight hours of shut-eye most of the population gets.

While for many, getting too little sleep can have some nasty consequences such as headaches and stomach problems, a tiny percentage of the population is able to thrive off just four to six hours, a phenomenon called "short sleeping."

Following President Donald Trump's annual physical exam, his physician told reporters on Tuesday that the president only sleeps four to five hours a night.

"He’s just one of those people, I think, that just does not require a lot of sleep," Dr. Ronny Jackson said.

It's something Trump has discussed before — while on the campaign trail, Trump said, "You know, I’m not a big sleeper. I like three hours, four hours, I toss, I turn, I beep-de-beep, I want to find out what’s going on."

Trump's not the only one who sleeps less than average. Corporate executives like PepsiCo CEO Indra Nooyi and even former President Barack Obama rarely — if ever — get what's considered a full night of sleep.

For many of us, getting too little sleep can have some nasty consequences such as headaches and stomach problems. But others are able to thrive off of just four to six hours of shut-eye, something called "short sleeping."

Short sleepers, a group The Wall Street Journal once called the "sleepless elite," need only a short amount of sleep every night instead of the average seven to eight hours. Scientists estimate those individuals make up only about 1% of the population.

It is not known whether Trump falls into this group. But there are a few characteristics that most of the short sleepers that have been identified thus far appear to have:

They tend to be more optimistic and upbeat than most people.

They tend to wake up early, even on vacation or weekends.

They tend to have a family member that is also a short sleeper. Since short-sleeping is linked to genetics, the behavior that accompanies it often runs in the family.

They tend to be physically active.

If they sleep longer than they need, they tend to feel groggy.

They say they tend to avoid caffeine or don't need it to feel energized.

This sleeping behavior is still a relatively new area of study, so a lot is unknown about short sleepers and their genetics. Having some of these traits doesn't necessarily mean you're a short sleeper, nor does not having some of these traits mean you're not.

The short sleep clinic

Even though it has no apparent negative health effects, short sleeping is considered a sleep disorder.

Although many people think they can get by with just four hours of sleep, most of them probably aren't true short sleepers — they're just chronically sleep deprived.

Ying-Hui Fu, a biologist and human genetics professor at the University of California, San Francisco started studying short sleepers in 1996, when a woman came into the lab asking them to investigate why her whole family woke up at extremely early hours every day. Fu investigated traits relating to that family and others who came into the clinic. Soon, she learned that there were three types of people: early risers, night owls, and people who are somewhere in between. Perhaps most importantly, she also learned that there were specific traits linked with all three types.

That work launched more than 20 years of studying these sleep behaviors to learn more about how people rest and how genetics play a role in that behavior.

"We know almost nothing about how sleep is regulated," Fu told Business Insider in 2015.

For now, most sleep research money goes toward treatments for sleeping disorders that deprive patients of sleep, which of course focus on helping people sleep more, not less.

But Fu thinks that belies how critical more research on short sleepers is. "Other than water and air, nothing is more important" than sleep, she said.

Abby Ross: Mother, doctor of psychology, marathoner, 'awaker'

When Abby Ross went to Fu's lab, she learned she was one of the rare short sleepers.

Ross has never needed what's considered a full night of sleep, and for years, she didn't know why she woke up feeling chipper and ready for the day, even after just four hours of sleep.

Ross decided to contribute to research at Fu's lab, giving blood and answering questions from psychologists and doctors from all over the world. But she doesn't know if she has the genes that have since been linked with being a short sleeper, since she agreed that any information the researchers gathered about her genes wouldn't be shared with her.

The lab's rationale for this, as they described it to Ross, was that if someone who came in with short-sleeping symptoms didn't have any of the already-identified short-sleeper genes, they could have another gene linked with the disorder that Fu's lab has yet to identify.

But Ross said the information she's gotten about herself so far is enough.

"I learned that what I have is truly a gift," she told Business Insider in 2015.

As long as she can remember, Ross said, she's been a short sleeper. When she was young, she'd be up early to get bagels and coffee with her parents. This early development of short sleeping habits is consistent with other short sleepers, who typically develop the habit sometime in childhood or as a young adult.

Ross got her undergraduate degree in three years by taking more classes than the average course load, which happened naturally for her. To her, an "all-nighter" wasn't a dreaded way to cram; it was just a regular night. Plus, Ross says, she's always had an easy time falling asleep, so if her body needed an hour or two, she'd take a nap then pick up right where she left off.

Ross went on to graduate school to study psychology and started a family.

"If I got up to feed the baby," she recalled, "I could stay up studying psychology."

She did it all, she said, by developing a respect for her body clock.

"It gave me permission to accept that if my husband goes to bed at 10:30, then I stay up," she said. "It's just the way it is."

Ross and her father Stan Kolber, who is also a short sleeper. Courtesy Abby Ross In true short-sleeper form, Ross has led an incredibly active life. Ten years ago, she ran 37 marathons in as many months. In one of those months, she did three marathons. Even since, she has often logged about five miles of walking and other activity on her Fitbit each day.

Ross puts her extra hours to good use, using them to do everything from catalog family photos to catch up with loved ones. Ross' 92-year-old father is also a short sleeper. For years, the two have emailed each other around 5 a.m. every morning to start their days.

For the most part, Ross has embraced her short sleeping gift, in all but name.

"I think the name is really weird" she said, since it sounds like people are referring to her height.

Instead of a short sleeper, Ross would like to be called an "awaker."

Recent developments

Being a short sleeper is, for the most part, genetic.

So far, Fu has pinpointed several genes connected to the disorder. One such gene is DEC2, which is known to affect our circadian rhythm, the biological process influenced by light and temperature that helps determine when we sleep and when we wake up. The other genes have yet to be published.

One of the main reasons Fu's lab hasn't quickly been able to publish much information on short sleepers is because it takes a long time — 10 years, she said — to publish the type of sleep-related paper she is planning. For these studies, researchers have to find and recruit short sleepers, which as only 1% of the population aren't easy to come by.

There isn't a ton of money going into sleep studies, which Fu said is the wrong approach, since understanding sleep habits could help people avoid diseases that are worsened by sleep deprivation.

"Instead of putting the fire out, let’s try to avoid fire," she said.

No official long-term health effects have been linked to being a short sleeper, though Fu said that is one concern her lab is looking into. For the most part, the people coming into Fu's lab are generally between 40 and 70 years old and in good health. Most stay active into their later years, and Fu said she's even had one volunteer in her lab who was 90 years old, so she hypothesizes that longevity could also be linked with being a short sleeper.

Ideally, Fu hopes to one day crack the code on how to become a short sleeper without being born with it. Then, maybe there will be more research focus to develop a gene therapy that can help people adapt to become short sleepers.

"I feel someday in the long-distance future, we can all sleep efficiently, and be healthy and smart," she said. "It's appealing to me."