When he was a college student in the 1960s, Carlin Black liked having a full class schedule. A chemistry major, minoring in both math and philosophy, he typically took 17 or 18 credits a semester.

Despite his workload, he was never in the library. While his friends knew they could find him at the gym most afternoons or playing cards in the dorm at night, they rarely saw him study. He'd go to bed on the early side, generally around 10 p.m.

But while his roommates blissfully slept through the night -- dreaming away nearly a third of the day -- Black was up again around midnight. "The house was quiet," he remembers fondly. "That was the most productive time of the day for me."

He'd usually go back to bed around 4 a.m. -- and then, like clockwork, wake up slightly before 6 a.m. There was never any need for an actual alarm clock. "I was the alarm clock," Black says. "That was my job in the house: make sure everyone else got up."

Four hours a night. That was the most sleep he ever got. And if there was a final exam to be crammed for or a paper to write, he could easily make do on one or two hours of sleep.

To this day, at the age of 74, Carlin says he never gets more than five hours ("and that's out of pure laziness"). When I ask him if he’s ever tried to sleep a full eight hours, he’s baffled. “I physically can’t sleep that much,” he tells me. “It’s just not how my body’s wired.”

Related: 3 Alarm Clock Alternatives

What would you do with an extra four hours a day? Train for a marathon? Learn a foreign language? Empty your inbox? Finally watch the movies stewing on your Netflix queue?

The options are endless. Time, after all, is the ultimate luxury; there never seems to be enough of it, so we’re forced economize, slicing up our hours and dividing them between conflicting responsibilities and activities. 

Unfortunately, for most of us, sleep is the one non-negotiable. To operate optimally, 90 percent of the human population needs somewhere between seven to nine hours a night, says Ying-Hui Fu, a human geneticist at the University of California-San Francisco. She estimates that a small segment of the population -- about 3 to 5 percent – needs less than 6.5 hours, while an even smaller sliver (Fu places it at less than one percent) can function normally on less than five hours.

The insidious effects of sleep loss are well documented. Research confirms that a lack of sleep impacts the body on a systematic level, disrupting everything from our metabolism to our immune system. Sleep loss has been linked to obesity, heart disease, reduced fertility, mood disorders, learning and memory problems and cancer. “It’s tied to everything,” says Fu.

And those are just the physical repercussions. Studies have also shown that extreme fatigue makes us, unsurprisingly, less alert. We're also more susceptible to cognitive errors of omission (which includes memory lapses), commission (which includes impulsive behavior) and find ourselves less able to handle small daily stressors. “Sleep deprivation is one of the most expensive problems for the world right now,” says Fu.

Not only does it contribute to a host of expensive medical problems, it makes us less efficient at work; Studies have shown that sleep deprivation impairs our ability to effectively make decisions, solve problems, effectively communicate and adapt to new situations; we become less flexible and less innovative.

So why is Black, along with the rest of the roughly one percent of the population that can operate on less than five hours a night, immune from all these serious side effects?

Common characteristics of short sleepers

"He's a short sleeper,” says Fu. In other words, Black has a series of genetic mutations that enables him to thrive on half the shut-eye most of us need. Back in 2009, Fu was part of a research team that discovered a genetic variation shared by a mother daughter pair who both went to bed past midnight and naturally woke up around 4 a.m. When Fu’s team replicated the mutation on a strain of mice, the rodents suddenly started needing less sleep, too.

Since 2009, Fu's team has found an additional two genes that may contribute to condition. It’s a trait that can run in families. Indeed, Black says his father rarely got more than five hours a night, and he believes his sister is also a short sleeper.

While Fu says that short sleepers may simply sleep more efficiently than the rest of us, that's just speculation. Sleep, for the most part, is still a giant enigma “It’s fascinating," says Fu. “We spend around a fourth to a third of our life asleep, and yet we don't know how it's regulated; we have very little idea about what sleep does for us.”

If you think you’re a 'short sleeper,' you’re probably lying to yourself.

Unfortunately, as of yet, we can't train ourselves to need less sleep.

While Fu dreams of the day she’ll be able to genetically alter her own DNA so she can become a short sleeper, she's accepted that for now, she needs at least seven hours.

The vast majority of us require eight solid hours a night. “Most of the variation in human sleep time is between six and eight hours. Once the sleep time gets above nine or below seven there are problems,” says David Dinges, who heads the Sleep and Chronobiology laboratory at the University of Pennsylvania. In other words, while your optimal sleep time rests on a sliding scale, if you are routinely getting less than six hours a night, there’s a 99 percent chance that you’re chronically fatigued.

Related: 12 Things Successful People Do Before Breakfast

How Much Sleep Do You Really Need?
Marissa Mayer
Image credit: Jolie O'Dell

After Dr. Fu’s research was published back in 2011, she received calls and emails from individuals who believed that they were short sleepers. Most, it turns out, were simply sleep deprived and either unaware or in denial. They were getting less than six hours of sleep a night, “but they didn't feel good,” says Fu. “They were drinking a lot of coffee, and after awhile they have to catch up on sleep.”

As a culture, sleep deprivation has increasingly become both a status symbol – not having time to sleep means you must be important – as well as evidence of a strong work ethic. This is especially true in the hyper-productive, ultra-competitive fish bowl that is the entrepreneurial community, where a good night’s sleep is often bastardized as a luxury reserved for the lazy. Marissa Mayer (CEO of Yahoo), Jack Dorsey (co-founder of Twitter and CEO of Square), Indra Nooyi (chairman and CEO of PepsiCo), Martha Stewart (chair of Martha Stewart Omnimedia) and Donald Trump (chairman of the Trump Organization) have all said that they average less than six hours of sleep a night. And that may be true. In addition to the extra amount of time, short sleepers, Fu's team has found, share characteristics that prime them for successful careers.

“They are very optimistic, very driven, very happy. A lot of them are very accomplished,” says Fu.  

These genetic findings are tantalizing; it's easy to see why Fu received calls for non-short sleepers who nonetheless were convinced they fit the bill. Who wouldn't want 25 percent more productive waking hours? 

A balancing act

“It’s popular to brag that you aren’t getting enough sleep,” Dinges says. “People will say, ‘I only need four hours of sleep a night,’ but in large scale sleep studies, we find that these people are incredibly rare; they represent less than one percent of the population.”

So how come it still seems like everyone and his mother is getting less than the recommended seven to eight hours, without batting an eyelash? We tend to overestimate our ability to go without sleep, while underestimating the amount of sleep we’re actually getting, says Dinges. Those 20 minutes you dozed off in the back of a taxi? That counts. So does an hour nap you took mid-flight. In the end, it’s the total amount of sleep you get in any 24 hour period that matters, not simply the sleep you average during the course of a night. “If you can get five hours of sleep at night plus a one hour nap, that's six hours of sleep, period” says Dinges.  In addition, he believes, most people don't advertise the fact that they catch up on sleep during their days off.

Still, despite these sneaky, overlooked catnaps, Dinges predicts that over 50 percent of the U.S. adult population is chronically sleep deprived. If you include the millions of people who have sleep disorders, he estimates that number jumps to 65 percent.

It's always going to be a balancing act.

As rational individuals, we are constantly tasked with making choices that leads to the most beneficial outcomes.

And sometimes, even when we know the medical and mental price, that elusive “most beneficial result” requires a sacrifice in the form of sleep.

In a recent study, Mark Showalter and Eric Eide, economists at Brigham Young University, challenged the conventional medical wisdom that success requires more, not less, sleep.

Related: Debunking 5 Common Myths About Sleep

While the medical recommendation for adolescents (10-17) is 8.5 to 9.25 hours a night, Showalter and Eide found that 16- to 18-year-olds perform better academically (based on data from a representative sample of 1,724 primary and secondary school students' standardized test scores) when they shaved about two hours off that recommended time.

Both authors emphasize that their definition of ‘optimal’ differs from a medical definition of the word. While most sleep researchers examine its effects on physical and mental health, Eide and Showalter's study looked at its effect on academic performance. "We're not looking at the effects of sleep on other health outcomes. We're simply describing the highest performing kids tend to sleep less than 9.25 hours that the medical literature had established as the guideline," says Eide.

"If you are sleeping you are not doing something else like studying or spending time with friends," Showalter explains. "There is an opportunity cost with sleeping that really hadn't been addressed in the medical literature."

While replicating a version of their study for adults is close to impossible – unlike teenagers, we're (thankfully) no longer subjected to mandatory standardized tests – Eide speculates that this same relationship is true for adults as well.

"If you were to look at earnings of adults as an outcome and you observed that people with high earnings tended to sleep less than people with lower earnings, that wouldn't be too surprising," he says.

When sleep deprivation flies under the radar

In other words, while eight hours a night may be the best option for your physical health, it's often not the best option for the health of your career. This, of course, is intuitive. "You have a deadline that needs to get done, you're going to stay up late to do it because staying up late is worth it," says Showalter. "In the real world, kids are faced with tradeoffs. And one of those is homework versus sleep or social activities versus sleep."

A catch-22.

Balancing the needs of our career with the needs of our health is complicated enough; so what happens when we don't have all of the necessary information required to make informed decisions? Is it possible to be chronically sleep deprived without realizing it?

Back in 2003, a team of researchers that included Dinges, conducted a two week experiment in which participants were divided into three groups: one got four hours of sleep a night, one got six hours a night, while a control group slept a luxurious eight hours a night. Every day each participant was given a series of psychomotor vigilance tasks (PVT), such as pressing a button when a bright spot appeared on a computer screen. On PVT tests, even slight delays in a participant's reaction time indicate lapses in alertness and wakefulness, says Dinges.

How Much Sleep Do You Really Need?
David Dinges, PhD

At the end of the experiment, participants who had logged eight hours a night displayed no impairment. But the reaction times for participants in the restricted sleep groups steadily declined over the two-week period. By day 14, the six-hour group's average score deteriorated to the point where it matched the average score for an individual who had been up for 24 hours straight.

The scary thing? Participants didn't recognize how impaired they had become.

By the end of the 14 days of sleep restriction, subjects in the four and six-hour sleep groups weren't aware that their performance had tanked. "The self-reports were all saturating exponential functions," says Dinges."They show some change after the first few days, but then they level off." In other words, while participants were able to recognize a decrease in their reaction time after a few days, eventually they began to believe they'd adapted to the restricted sleep schedule; their reaction times continued to worsen, but they were unable to recognize the continual decline.

"They'd say: 'I've adapted. I'm a little tired, but I'm alright," Dinges recalls. "That's what's so dangerous about allowing a deficit to build up. You never experience a true recovery and so the brain never resets. It begins to think the sleep deprived state is normal."

Related: Yes, You Can Sleep In and Still Be Successful

And when it comes to our ability to handle sleep deprivation, well…some of us can do it better than others. "You can have two people who need exactly the same amount of sleep a night," Dinges says. "Except one of them is extremely vulnerable to sleep loss and has cognitive deficits within one night of reduced sleep, while the other one doesn't experience cognitive deficits until five nights on a reduced sleep schedule."

This resistance, Dinges says, has nothing to do with the amount of sleep a person needs a night. You may need nine hours a night, but still be incredibly resistant to cognitive deficits that accompany sleep loss. Or you may only need six hours a night, but face extreme cognitive deficits if you try and get away with just five.

It's a catch-22 of sorts. The less you sleep and the more cognitively impaired you become, the less qualified you are to recognize that you are cognitively impaired in the first place. Suddenly, it becomes easier to buy into the fiction that fatigue can be fought with caffeine, bright lights and sheer willpower.

So how can you determine your ideal sleep schedule?

While the majority of us need somewhere between seven and nine hours, that's still a large window.

In addition, optimal sleep time – the hours at which we get our best rest – differs dramatically throughout the population. Back in 1996, a director of a sleep clinic approached Dr. Fu with a special case; one of his patients, a 69-year-old woman, exhibited an unusual sleep cycle; like clockwork, she would get incredibly tired at 8 p.m., and consistently wake up around 4 a.m. each morning.  Worried after noticing the same unusual sleep pattern in her granddaughter, the woman visited a host of doctors; many concluded that she was suffering from undiagnosed depression. Why else would she be up at 4 a.m. every morning?

How many hours do you really need a night?

In reality, "She has no health problems whatsoever," says Dr. Fu. "She is completely normal, except that her sleep schedule is shifted forward."

Intuitively, most of us know if we are an early bird or a night owl. And despite the myriad of articles promising that becoming a "morning person" simply requires following a rounded-off number of tips, it's largely genetic. You're either born a morning person, or you're not. "Our bodies are wired in a certain way," says Dr. Fu. "All of our bodily functions are regulated by our circadian rhythm. Your body temperature has a rhythm, your heartbeat has a rhythm…you can wake up before your body is ready, but a lot of your body's functions aren't fully awake yet." (This lag in cognitive functioning is known as sleep inertia, that period between waking and being fully awake when you feel groggy.)

So how to determine the number of hours you need a night as well as when you should be sleeping them? A common recommendation is to simply pay attention to your body the next time you go on vacation: avoid all stimulants like caffeine; go to bed when you feel tired; turn off your alarm clock, and see when you wake up naturally. But both Dr. Fu and Dinges caution that to determine your optimal sleep schedule, you may need to catch up on sleep first. If you are extremely sleep deprived going into your vacation, "two days is not going to be enough for you to recover," says Fu. Instead, she recommends resting for two or three days before paying attention to your body's natural clock.

Related: Beware! Is a 9-to-5 Schedule Turning Employees Into Zombies?

How Much Sleep Do You Really Need?
Dr. Ying-Hui Fu and her research team

"Most people can learn to live on the schedule that's optimal for them," says Dinges. Most of us have the same amount of productive hours, we just are on slightly altered schedules. Dr. Fu finds that for many people, the most difficult part is not work, but coordinating their schedule with that of their partner's. This can be especially hard for people whose schedule is dramatically shifted forward or backward. "They have to marry someone with the same sleep schedule," laughs Fu. "Otherwise, when they want to go to bed at seven or eight, their spouse wants to go out and do things."

So, all you sleepers out there still looking for love; the next time you're out on a date, perhaps consider if you're sleep schedules are compatible. Or perhaps find yourself a short sleeper instead.

Black says his unusual sleep schedule never bothered his wife. He worked long days and could still help take care of the kids. "I don't think she minded waking up to a hot breakfast every morning."