We live in a sleep-deprived nation. Although the Centers for Disease Control and Prevention (CDC) recommends adults get seven to nine hours of shut-eye per night, one-third of Americans rack up less than that, according to a February 2016 CDC study.
For some of us, spending that long between the sheets can feel impossible. Whether you work the night shift, have a new baby who's up at all hours, stay up late studying or have trouble falling asleep because of stress, you might go days, weeks or even months without getting solid rest.
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For others, dozing off for only five or six hours is your MO. Maybe you feel fine and don't think you need more sleep, or perhaps your schedule is chock-full and spending an extra hour or two with the Sandman isn't a priority.
Since so many of us are falling short when it comes to sleep, we decided to investigate what exactly happens during those seven to nine hours, how to determine the right amount of zzzs for you and what can happen to your brain and body if you're not reaching your target sleep quota.
Stay Up Now, Pay Later
After sleeping fewer than seven hours, you might notice that you're yawning more than usual, you're not on your A-game or you have a short fuse. But hey, it's not the end of the world, right?
Well, many of the health issues that can arise from skimping on sleep don't present as an immediate cause-and-effect.
"Insufficient sleep wears out your health engine earlier, but it can take many years before you may start to see medically significant symptoms," says Michael Twery, PhD, director of the National Center on Sleep Disorders Research. "People who are sleep-deprived also tend to develop clinical medical issues later in life as years of sleep inefficiency eventually catch up with them."
Let's say after a lifetime of sleeping five hours a night you have a heart attack.
"Although the laboratory results indicate that heart attack was the cause of death, researchers are starting to connect the dots on how not sleeping enough increased the inflammation and atherosclerosis that contributed to your heart attack," Twery says. "Or if you die from cancer, the reason you had a greater risk of disease was because your immune system wasn't working at its best due to lack of sleep."
In other words, if you're not getting adequate shut-eye right now but aren't suffering, that doesn't mean you're off the hook. Rather, your poor sleep might just not have caught up with you yet.
"When researchers study large populations, on average people who report sleeping less than seven hours on a regular basis are at increased risk of poor health, disease and all causes of mortality," Twery says. "Although too little sleep is not the smoking gun connection [and some people who get paltry sleep don't suffer negative health consequences], science argues that seven to nine hours is best for most people."
It's also important to note that there is a link between health issues and sleeping too much. According to Johns Hopkins Medicine, snoozing longer than nine hours is associated with type 2 diabetes, heart disease, obesity, depression and mortality.
What Can Happen if You’re Starved for Sleep?
First off, don't worry if you occasionally stay up late going out with friends or burning the midnight oil at work, or if you have a sleepless night here or there when you're sick or jet-lagged.
"One night of sleep deprivation is not a big deal," Twery says. "Our bodies can always stand some deviation from the ideal."
But habitually cutting your slumber short does take a toll. Here are some of the health risks linked to too-little sleep:
An October 2018 review in BMJ Open Sport & Exercise Medicine found that people who habitually slept fewer than seven hours a night were more likely to have a higher BMI and be obese.
"When you don't get enough sleep, your body reduces the release of leptin, a hormone that helps suppress appetite and encourages the body to use energy," says sleep specialist Michael Breus, PhD, author of The Power of When.
The researchers also discovered that too-little sleep increases levels of the hormone ghrelin, which stimulates appetite and can lead to weight gain. In addition, they point out that people who aren't spending ample time in the sack have less energy for physical activity during the day. And obesity puts you at greater risk for sleep disorders, thus perpetuating the problem.
A November 2016 study in Oman Medical Journal found an association between poor sleep and type 2 diabetes.
"Sleep-deprived middle-aged and older folks are twice as likely to be diagnosed with type 2 diabetes than well-rested people," Breus says. "That's because sleep helps regulate glucose and metabolism. Sleep deprivation also leads to a spike in cortisol, which can make cells more resistant to insulin."
Insufficient sleep contributes to atherosclerosis, a build-up of plaque inside your arteries that can restrict blood flow and lead to heart disease, according to a January 2019 study in the Journal of the American College of Cardiology.
Meanwhile, a July 2020 study in Psychosomatic Medicine discovered that people who spend minimal time in dreamland also have higher blood pressure.
"Over time, this leads to more plaque blocking the coronary arteries," Breus says.
So it's no surprise that a September 2019 study in the Journal of the American College of Cardiology revealed that short sleepers have a 20 percent higher risk of heart attack.
In a September 2015 study in Sleep, people who slept six hours a night or fewer were four times likelier to catch a cold. In fact, researchers determined that sleep was the single greatest predictor of illness — more influential than age, stress level, race, education, income or being a smoker.
"Poor sleep reduces the production and release of cytokines," Breus says. These proteins help the immune system marshal a defense against threats, per the Cleveland Clinic. "When you're sleep-deprived, your body isn't able to act as quickly, or send as much cytokines, to counter any problems it might run into," Breus says.
Lack of sleep also reduces the release of infection-fighting antibodies, per the Mayo Clinic.
Not only can lack of sleep increase the likelihood that you'll get sick after you've been exposed to a virus, but it can also lengthen the amount of time it takes you to recover from illness, according to the Mayo Clinic.
Reduced Cognitive Function
In the short-term, skimping on sleep impairs your ability to focus, retain information and synthesize new ideas; you're also more likely to make mistakes, according to the Division of Sleep Medicine at Harvard Medical School.
"When you get under six hours of sleep, your reaction time slows by 30 percent and gets exponentially worse the less sleep you get," Breus says.
That's because your brain undergoes an important recharging process when you're zonked out.
"During deep sleep, your neurons and the cells in your cortex activate and then inactivate in a pattern we call slow waves," Twery says. "This slow wave movement allows cerebral spinal fluid to seep into the cortex and flush out toxins that gum up the brain."
If you short-change your deep sleep, these toxins aren't adequately cleared out. As a result, your thinking won't be as sharp the next day. Even more alarming? According to an April 2018 report from the NIH, one of the toxic proteins that are swept away during deep sleep is associated with Alzheimer's disease.
Not catching a full 40 winks can lead to irritability and anxiety.
"It undermines your ability to properly regulate your stress hormones and your cortisol levels go up," Twery says.
Lack of sleep can also take a toll on your social and family life.
"It affects our emotional reserve and adds to interpersonal problems," Twery says. "When we are sleep-deficient and our cortex — the thinking part of the brain — is not well rested, we are not able to control the more primitive parts of the brain."
Translation: You're more likely to lash out, get defensive and make bad decisions — which is a recipe for rocky relationships.
So, How Many Hours of Sleep Do You Actually Need?
Sleep is not one-size-fits-all; the amount you require depends on your age, lifestyle and genetics.
"From birth until age 18 to 20, we are going through a process of neurodevelopment and need more than seven to nine hours of sleep," Twery says.
Seniors may require less shut-eye at night if they regularly nap during the day, Twery says.
Your menstrual cycle can also influence sleep — women might need more rest before and during their period. Pregnant women and high-performance athletes should log extra time under the covers, too.
But for most of us, our ideal sleep time comes down to DNA.
"It is genetically determined based on your chronotype, or biological clock," Breus says.
It varies from person to person, which is why you might feel energetic after seven hours of sleep while your partner is grumpy if they get less than eight.
"The idea that everyone needs eight hours of sleep is a myth — sleep is personalized," Breus says. "I sleep for six hours and 15 minutes every night and I'm a sleep expert."
In an October 2019 study in Science Translational Medicine, researchers identified a "short sleeper" gene. People with this genetic mutation can conk out for just four to six hours without any negative side effects. "However, this variant is incredibly rare, affecting only half a percent of the population," Breus says.
So what about people who claim they only need five hours of sleep and feel fine in the morning? Consider this a, um, wake-up call.
If you're a coffee-holic, caffeine consumption might be making you feel more awake than you actually are.
"There is also a survival mechanism in your brain that tells you to push through your tiredness, even if you are sleep-deprived — so that might be kicking in," Breus says. "But I guarantee that, unless someone has a rare genetic variation, they will sleep longer than five hours if they were in a sleep lab."
Translation: You should probably hit the hay earlier.
Use This Formula to Figure Out the Right Amount of Sleep for You
According to Breus, most people go through five sleep cycles each night. Each cycle lasts between 80 and 120 minutes, with an average length of 90 minutes. Since it takes the average person 7.5 hours to go through five 90-minute sleep cycles, Breus suggests using 7.5 hours at your starting sleep set-point.
Now, take your normal wakeup time and go back 7.5 hours. Make that your bedtime. For example, if you need to wake up at 6:30 a.m., you should go to bed at 11:00 p.m. Stick with this bedtime for 10 days. If, after 10 days, you are waking up without an alarm clock and feeling refreshed, then you've pinpointed the right amount of sleep for you.
If you still need an alarm in order to wake up, it might indicate that your sleep cycles last longer than 90 minutes. Move up your bedtime by half an hour and after 10 days see if you are waking up naturally. Continue experimenting in 10-day intervals until you home in on your ideal bedtime.
You can try Breus's online Sleep Calculator to refine your sleep set-point even more. But basically, if you are waking up without an alarm feeling alert and energized, then you are good to go.
If you still feel sluggish in the morning after sleeping for nine hours, it could signal a sleep disorder or medical condition that's preventing you from getting quality rest. In that case, it's a good idea to talk to your doctor.
6 Tips to Get More Sleep
Try these research-driven strategies to increase your zzzs.
1. Stick to a sleep schedule by going to bed and waking up at the same time every day. This will help you fall asleep faster and more easily, while also supporting your overall health, Twery says.
2. Skip caffeine in the afternoon. "Drinking caffeinated beverages all day is the most common source of a sleep disorder," Twery says.
3. If you're on any medications, talk to your doctor about whether they might be interfering with your sleep. Twery says meds are another frequent culprit of sleep issues. Wondering whether any of the prescription or over-the-counter drugs you take could be causing you to toss and turn? This list from the National Sleep Foundation is a good starting place.
4. Make sure your bedroom is dark, quiet, cool and screen-free.
5. Get plenty of sunlight — particularly in the morning. "Sunshine is a powerful tool to regulate our circadian rhythm so that our body is getting the wake and sleep signals we need," Twery says.
6. Avoid alcohol and large meals close to bedtime — these can interrupt your slumber.
So, How Bad Is It Really to Get Less Than the Recommended Amount of Sleep?
According to Breus, the answer lies in how much sleep you personally require — and how much less than that you're actually getting.
"As long as you feel fine, I don't think it's bad at all," Breus says. "I can't remember the last time I got seven hours of sleep in a night."
But if you're snoozing less than your body and brain needs in order to recover and reboot, that's a different story.
"For instance, if you calculated that you require seven hours of sleep and are only getting five, that's pretty bad," Breus says. And if you feel lousy, that's a red flag.
Still, it's not always obvious when you're overtired. Not only can caffeine trick you into thinking you've gotten enough rest, but eventually feeling sluggish will become your new normal and you won't realize that you're not as on-the-ball as you could be.
So your safest bet may be to stick to the seven- to nine-hour recommendation.
"The science has revealed that by not getting enough sleep, we are short-changing the opportunity to achieve our best health trajectory over our lifetime," Twery says.
- BMJ Open Sport & Exercise Medicine: "Sleep deprivation and obesity in adults: a brief narrative review"
- Oman Medical Journal: "Habitual Sleep Deprivation is Associated with Type 2 Diabetes: A Case-Control Study"
- Journal of the American College of Cardiology: "Sleeping Less Than Six Hours a Night May Increase Cardiovascular Risk"
- Psychosomatic Medicine : "Associations Between Objective Sleep and Ambulatory Blood Pressure in a Community Sample"
- Journal of the American College of Cardiology: "Sleep Duration and Myocardial Infarction"
- Sleep: "Transcriptional Signatures of Sleep Duration Discordance in Monozygotic Twins"
- Sleep: "Behaviorally Assessed Sleep and Susceptibility to the Common Cold"
- NIH: "Sleep deprivation increases Alzheimer’s protein"
- Division of Sleep Medicine at Harvard Medical School: "Sleep, Learning and Memory"
- Science Translational Medicine: "Mutant neuropeptide S receptor reduces sleep duration with preserved memory consolidation"
- Johns Hopkins Medicine: "Oversleeping: Bad for Your Health?"
- National Sleep Foundation: "How Medications May Affect Sleep"
- CDC: "1 in 3 adults don’t get enough sleep"
- CDC: "How Much Sleep Do I Need?"
- Cleveland Clinic: "Cytokines"
- Mayo Clinic: "Lack of sleep: Can it make you sick?"
Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.