Celebrity hair stylist Kristan Serafino’s jam-packed schedule forced her to put a good night’s sleep on the backburner. Serafino—who has styled Matthew McConaughey, Meg Ryan—would nab only four hours of shut-eye each night, and lean on props at photo shoots to keep herself from collapsing. “Chugging coffee was a big help,” she recalls.Her wake-up call came when she conked out at the airport gate while waiting for—and nearly missing—her flight to a photo shoot. After that, Serafino decided to make sleep a priority, and now hits the sheets earlier to log six to eight hours of z’s each night. “Now that I have better control of my sleep patterns, I wake up in the mornings with more energy, and I am down to one cup of coffee a day,” she says.What Serafino doesn’t realize is that by increasing her nightly slumber from a paltry four hours of sleep to at least six, she may have also prolonged her life. That’s because not getting enough shut-eye can actually shorten your life span. Yep, you heard that right.The health consequences of too little sleepEven though sleep researchers are still trying to unravel why, exactly, we sleep—it seems to have restorative, healing powers and helps us sort and file away our daily memories—there’s no doubt that we can’t live without it.“You can live longer without food and water than you can without sleep,” says Lisa Shives, M.D., medical director of Northshore Sleep Medicine in Evanston, Illinois.STUDY: Sleep Boosts BeautyConsistently not getting enough sleep doesn’t just leave you a dazed, moody and forgetful former version of yourself, chronic sleep deprivation can also put you a risk for a whole host of health problems ranging from diabetes to hypertension and obesity—all of which can affect longevity, according to Sonia Ancoli-Israel, Ph.D., director of the Gillin Sleep and Chronomedicine Research Center and professor of psychiatry at the University of California, San Diego.Too much sleep hurts health, tooUnlike Serafino who trudged through the day on little sleep, beauty editor Courtney Dunlop was convinced she couldn’t function without nine hours of slumber each night. “I used to say that I prefer 10 hours of sleep, but that I try for at least nine,” recalls Dunlop. “The thing is, I was always tired. All day long—tired.”Dunlop’s tipping point came when she got a new job that required her to show up at the office by 9 a.m. rather than the 10 a.m. start time she was used to at her previous job. “It was unrealistic to go to bed earlier than I already was, so I started getting seven-and-a-half hours of sleep at night.”Surprisingly, Dunlop felt more alert, with enough extra energy to work out in the evenings. “I used to oversleep almost every day,” she says. “I’m not saying I suddenly jump out of bed all cheery now, but it’s not as hard anymore.”MORE: The Beauty Benefits of Sleep Dunlop did her energy, and her health, a favor by scaling down her sleep schedule. New sleep research shows there is a U-shaped relationship between slumber and longevity, meaning that getting too little—or too much—sleep will raise your mortality rate. Numerous studies have come to the same conclusion: People who self-reported sleeping more than seven-and-a-half hours a night or less than six to six-and-a-half hours a night had an increased risk of death.While not logging enough snooze time has a clear and direct affect on your health, “getting too much sleep—a condition called hypersomnia—isn’t a problem on its own per se,” says Dr. Shives. It’s the fact that oversleeping can be a red flag for an underlying health condition, such as obstructive sleep apnea—a dangerous disorder that causes people to stop breathing momentarily during sleep—and depression.The optimal amount of sleepSo how much do you need to nab for a longer life? For years people have been told that when it comes to sleep, eight is the magic number, but that’s actually the upper range. Although most sleep experts recommend getting seven to eight hours of sleep on average, several studies show that getting around seven to seven-and-a-half hours a night lowers your mortality risk.But don’t fret if you get more or less—it’s just an average, and you’re not going to die from not getting enough z’s one night. Also, keep in mind that many sleep studies are self-reported, meaning the study participants estimate how much shut-eye they believe they logged. But people aren’t always so good at ball-parking the amount of sleep they actually got.Although you can clock more snooze time if you feel like you’re not getting enough sleep, as Serafino did—or like Dunlop, cut back if you’re overdoing it—you can’t train yourself to get by on less (or more) sleep than your body actually needs.“Although the average adults needs seven to eight hours of sleep, some adults are longer and shorter sleepers,” says Dr. Ancoli-Israel. “This is genetically determined, so you can’t change it or teach yourself to need more or less of what your body naturally requires.”BEAUTY AHA: Sleep Eight Hours to Stay Healthy and HappySo how can you determine exactly how much sleep your body needs? “It’s the amount that allows you to function at an optimal level during the day, meaning you’re not dozing off or fighting to stay awake in the movie theater or during a boring meeting,” she says. “You should be able to get through the day without having to drink coffee or energy drinks.”While getting the right amount of shut-eye may be a Herculean task for some, if you had to choose between staying up to watch late night reruns of “SNL” and upping your chances for a longer, healthier life simply by getting a good night’s sleep, you already know which one to pick.