In our 24/7 culture, sleep loss is a major problem. Back in 1942, we averaged almost 8 hours of sleep a night — now that's down to 6.8. (Seven to 9 hours per night are what's generally recommended.) Almost 40% of Americans get less than 7 hours of sleep a night, a recent Gallup poll found, and an estimated 70 million Americans have a sleep disorder.
Everyone knows that it's important to get enough sleep — but you may not realize just how many things can go wrong when you don't.
Here are 25 unfortunate risks of partial and total sleep deprivation, some more common than others.

1. Irritability

"Complaints of irritability and [emotional] volatility following sleepless nights" are common, a team of Israeli researchers observed. They put those complaints to the test by following a group of underslept medical residents. The study found that the negative emotional effect of disruptive events — things like being interrupted while in the middle of doing something — were amplified by sleep loss.
Source: Sleep, 2005

2. Headaches

Scientists don't yet know exactly why sleep deprivation leads to headaches — but it's a connection doctors have noticed for more than a century. Migraines can be triggered by sleepless nights, and 36 to 58% of people with sleep apnea wake up with "nondescript morning headaches."
Source: Headache, 2003; Headache, 2005

3. Inability to learn

Sleepiness has long been an issue among adolescents. One study of middle school students found that "delaying school start times by one hour, from roughly 7:30 to 8:30, increases standardized test scores by at least 2 percentile points in math and 1 percentile point in reading."
But it's not just kids. Short-term memory is a crucial component of learning, and sleep deprivation significantly impaired the ability of adult volunteers to remember words they'd been shown the day before. In another study, researchers found that while people tend to improve on a task when they do it more than once, this isn't true if they are kept awake after they try it the first time — even if they sleep again before doing it again.
Source: Nature, 1999; Nature Neuroscience, 2000; Education Next, 2012

4. Weight gain

People who are underslept seem to have hormone imbalances that are tied to increased appetite, more cravings for high-calorie foods, a greater response to indulgent treats, and a dampened ability to control their impulses — a very dangerous combination. It's true that you burn more calories when awake, but not nearly enough to cancel out the many excess calories you consume when exhausted.
Source: Journal of Clinical Endocrinology & Metabolism, 2012; PLOS Medicine, 2004; Nature Communications, 2013; PNAS, 2013

5. Poor vision

Sleep deprivation is associated with tunnel vision, double vision, and dimness. The longer you are awake, the more visual errors you'll encounter, and the more likely you are to experience outright hallucinations.
Source: International Journal of Occupational Medicine and Environmental Health, 2010

6. Heart disease

When researchers kept people awake for 88 hours, their blood pressure went up — no big surprise there. But even subjects who were allowed to sleep for 4 hours a night had an elevated heart rate when compared to those getting 8 hours. Concentrations of C-reactive protein, a marker of heart disease risk, increased in those fully and partially deprived of sleep.
Source: Journal of the American College of Cardiology, 2004; PLOS ONE, 2009; Sleep Medicine Reviews, 2012

7. Slowness

Your reaction time is severely impeded when you don't get enough sleep. When researchers gave West Point cadets two tests that require quick decision-making, some were allowed to sleep between the tests, while others were not. Those who had slept did better the second time — those who had not did worse, and their reactions slowed down. A study in college athletes found similar results.
Source: Sleep, 2009; Asian Journal of Sports Medicine, 2012

8. Infection

You know that great thing your immune system does, where when you get an open wound of some kind it doesn't always get infected immediately? Prolonged sleep deprivation and even one night of sleeplessness can impede your body's natural defenses against microorganisms.
Source: American Journal of Physiology, 1993; The FASEB Journal, 1996

9. Economic risk-taking

Planning to make some changes to your portfolio? You might want to make sure you're well-rested. "A single night of sleep deprivation evoked a strategy shift during risky decision making such that healthy human volunteers moved from defending against losses to seeking increased gains," researchers concluded.
Source: The Journal of Neuroscience, 2011

10. Overproduction of urine

When people sleep, the body slows down its normal urine production. This is why most people don't have to pee in the night as much as they do during the day. But when someone is sleep deprived, this normal slowdown doesn't happen, leading to what researchers call "excess nocturnal urine production." This condition may be linked to bed wetting in children and, in adults, it's tied to what's called nocturia — the need to use the bathroom many times during the night.
Source: American Journal of Physiology, 2010; American Journal of Physiology, 2012

11. Distractedness

Having trouble paying attention to what you're reading or listening to? Struggling with anything that requires you to truly focus? "Attention tasks appear to be particularly sensitive to sleep loss," researchers have noted. If you want to stay alert and attentive, sleep is a requirement. Otherwise, you enter "an unstable state that fluctuates within seconds and that cannot be characterized as either fully awake or asleep," and your ability to pay attention is variable at best.
Source: Archives of Italian Biology, 2001; Seminars in Neurology, 2009

12. Less effective vaccines

Vaccines work by spurring your body to create antibodies against a specific virus. But when you don't sleep, your immune system is compromised, and this doesn't work quite as well. In one small study, 19 people were vaccinated against Hepatitis A. Ten of them got 8 hours of sleep the following night, while the rest pulled an all-nighter. Four weeks later, those who had slept normally had levels of Hepatitis A antibodies almost twice as high as those who'd been kept awake.
Another study found that a sleepless night did not have a long-term effect on immunity after a flu vaccine, it concludes that the effect might be specific to certain diseases. "Sleep should be considered an essential factor contributing to the success of vaccination," the Hep A researchers wrote.
Source: Psychosomatic Medicine, 2003; Journal of Immunology, 2011; BMC Immunology, 2012

13. Impaired speech

Severe sleep deprivation might make you sound like a bumbling idiot — much like having way too much to drink. "Volunteers kept awake for 36 hours showed a tendency to use word repetitions and clichés; they spoke monotonously, slowly, [and] indistinctly," one study noted. "They were not able to properly express and verbalize their thoughts."
Source: Sleep, 1997; International Journal of Occupational Medicine and Environmental Health, 2010

14. Colds

If you're wondering why you're sick all the time and seem to pick up every bug that travels around the office, it's probably because you're not getting enough sleep. When a group of 153 people were exposed to a common cold, those who had gotten less than 7 hours of sleep in the two weeks prior were almost 3 times more likely to get sick than those who'd had 8 or more hours of sleep. How well you sleep is also a factor – those who had spent 92% of their time in bed actually asleep were 5.5 times more likely to catch a cold than those who had been peacefully slumbering 98-100% of the time they were in bed.
Source: Archives of Internal Medicine, 2009

15. Gastrointestinal problems

One in 250 Americans suffer from Inflammatory Bowel Disease (IBD), and sleep deficiencies make its symptoms much worse. Regular sleep loss also makes you more likely to develop both IBD and inflammatory bowel syndrome, which affects an estimated 10-15% of people in the U.S. And patients with Crohn's disease were twice as likely to experience a relapse when they weren't getting enough sleep.
Source: World Journal of Gastroenterology, 2013

16. Car accidents

Drowsy driving is often compared to drunk driving: You really shouldn't do either. "Motor vehicle accidents related to fatigue, drowsy driving, and falling asleep at the wheel are particularly common, but often underestimated," one review concluded. Pilots, truck drivers, medical residents, and others required to stay awake for long periods of time "show an increased risk of crashes or near misses due to sleep deprivation."
Source: Seminars in Neurology, 2009

17. Depleted sex drive

Testosterone is an important component of sexual drive and desire in both women and men. Sleeping increases testosterone levels, while being awake decreases them. Sleep deprivation and disturbed sleep, consequently, are associated with reduced libido and sexual dysfunction, and people suffering from sleep apnea are at particular risk.
Source: American Journal of Obstetrics and Gynecology, 2007; Behavioral Brain Research, 2009; Journal of Sexual Medicine, 2009; Sleep Medicine, 2010; Brain Research, 2011 

18. Pain

People in pain — especially those suffering from chronic pain — tend not to get enough sleep. This makes sense: Pain can wake you up in the night and make it hard to fall asleep in the first place. But recently, researchers have begun to suspect that sleep deprivation may actually cause pain or at least increase people's sensitivity to pain. One study found that after research subjects were kept awake all night, their pain threshold — the amount of painful stimulus they were able to endure — was lower.
Source: Journal of Sleep Research, 2001; Sleep Medicine Reviews, 2006

19. Diabetes

Being awake when your body wants you to be asleep messes with your metabolism, which in turn increases your risk for insulin resistance (often called "pre-diabetes") and type 2 diabetes. "Interventions to extend sleep duration may reduce diabetes risk," one study in adolescents concluded. And four large studies in adults found a strong association — though not a cause-effect relationship — between regular sleep loss and the risk of developing diabetes, even after controlling for other habits that might be relevant.
Source: Journal of Applied Physiology, 2005; Sleep, 2012; Annals of Internal Medicine, 2012

20. Sloppiness

Most people notice that when they're sleepy, they're not at the top of their game. One study found that one sleepless night contributed to a 20-32% increase in the number of errors made by surgeons. People playing sports that require precision — shooting, sailing, cycling, etc. — also make more mistakes when they've been awake for extended periods of time.
Source: The Lancet, 1998; Physiology & Behavior, 2007

21. Cancer

Scientists are just beginning to investigate the relationship between sleep and cancer, and different kinds of cancer behave differently. But since disrupted circadian rhythm and reduced immunity are direct results of sleep deprivation, it's no surprise that preliminary research seems to indicate that people who don't get enough sleep are at increased risk for developing certain kinds of cancer, most notably colon and breast cancers.
Source: Journal of the National Cancer Institute, 2003; Pathologie-biologie, 2003; Cancer, 2011; AAOHN Journal, 2011 

22. Memory problems

Sleep disruptions in the elderly can lead to structural changes in the brain that are associated with impaired long-term memory — and sleep-related memory deficits have been observed in the general adult population as well. As early as 1924, researchers noticed that people who slept more forgot less. Poor sleep and not enough of it have also been linked to higher levels of β-Amyloid, a biomarker for Alzheimer's.
Source: Cell Signal, 2012; Nature Neuroscience, 2013; JAMA Neurology, 2013

23. Genetic disruption

A 2013 study shed some light on why sleep is tied to so many different aspects of our health and wellness. Poor sleep actually disrupts normal genetic activity. After one week of sleeping less than 6 hours per night, researchers found that more than 700 genes were not behaving normally, including some that help govern immune and stress responses.
Some genes that typically cycle according to a daily (circadian) pattern stopped doing so, while others that don't normally follow a daily pattern began doing so. What does this mean? Just one week of less-than-ideal sleep is enough to make some of your genetic activity go haywire.
Source: Proceedings of the National Academy of Sciences, 2013

24. Unhappiness and depression

In a classic study led by Nobel Prize-winning psychologist Daniel Kahneman, a group of 909 working women kept detailed logs of their moods and day-to-day activities. While differences in income up to $60,000 had little effect on happiness, a poor night's sleep was one of two factors that could ruin the following day's mood. (The other was tight deadlines at work.)
Another study reported higher marital happiness among women with more peaceful sleep, although it's hard to say whether happy people sleep better, better sleep makes people happier, or — most likely — some combination of the two. Insomniacs are also twice as likely to develop depression, and preliminary research suggests that treating sleep problems may successfully treat depressive symptoms.
Source: Science, 2004; Behavioral Sleep Medicine, 2009; Journal of Affective Disorders, 2011

25. Death

Many health problems are associated with sleep deprivation and poor sleep, but here's the big one: People who consistently do not get 7-8 hours of sleep are more likely to die during a given time period. Put more simply: We all die eventually, but sleeping too little — or even too much — is associated with a higher risk of dying sooner than you otherwise might. 
Source: Sleep, 2010; Sleep Medicine Reviews, 2010