Sleep deprivation and a disrupted sleep schedules have been linked to increased risk for a number of cancers, most notably colon and breast cancers.Poor sleep quality is strongly correlated to chronic skin problems, according to research from the University of Wisconsin. Studies have also shown that when skin is damaged by the sun or other factors, it doesn't heal as well in poor sleepers, so those people wind up showing more signs of skin aging.People who don't get enough sleep have a harder time resisting high-calorie foods, more cravings for unhealthy meals, and difficulty controlling their impulses. Researchers think hormonal imbalances that result from sleep deprivation are responsible for this, since those imbalances are linked to a high body mass index and obesity.Researchers have found that sleep-deprived young adults are less likely to connect socially with other people, and that people who report poor sleep also tend to say they're more lonely. To make things worse, people who feel lonely don't tend to sleep as well, which can lead to a sort of vicious cycle.Sleepiness has long been a problem for students. Delaying school start times an hour for middle school kids significantly increases standardized test scores, and research shows this has an even bigger effect on teens, who naturally tend to be night owls. But it's not just kids — sleep deprivation also wrecks adults' short-term memory. Several studies have shown that sleep-deprived adults have more difficulty remembering words they've learned and have a harder time improving at newly learned skills.Sleep disruptions in the elderly can lead to structural changes in the brain that are associated with impaired long-term memory. 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.Several studies have shown that sleep helps cleanse the brain of the beta-amyloid protein that can build up while you are awake. That protein is strongly associated with Alzheimer's disease. Researchers say that a lack of sleep can lead to a vicious cycle, since the more beta-amyloid there is in the brain, the harder it is to get to a cleansing deep-sleep state. People with more disrupted sleep schedules tend to have more beta-amyloid built up.Plenty of evidence shows sleep deprivation has a negative effect on the heart. When researchers kept people awake for 88 hours, their blood pressure went up (no big surprise there). Even subjects who were allowed to sleep for four hours per night showed an elevated heart rate when compared to those who got 8 hours. Concentrations of C-reactive protein, a marker of heart disease risk, also increase in people who are fully or partially deprived of sleep.People feel irritable after sleepless nights (as we've all experienced at some point), and research also shows that people get more distressed by common circumstances like interruptions at work when they are tired.Sleep deprivation is associated with tunnel vision, double vision, and perceived dimness. The longer you are awake, the more visual errors you'll encounter, and the more likely you are to experience outright hallucinations.Your reaction time is severely impeded when you don't get enough sleep. Studies have shown that college athletes and West Point cadets all did worse on decision-making tests and had slower reactions while tired.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.You know those great things your immune system does when you get a wound but don't immediately get an infection, or you come near a sick person but don't get ill yourself? Prolonged sleep deprivation and even one night of sleeplessness can impede your body's natural defenses against infection. Sleep deprivation also seems to make newly received vaccines less effective.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. Sleep-deprived people are almost three times as likely as well-rested people to catch a cold, according to one study.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. People suffering from sleep apnea are at particular risk.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, the results showed, 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 or good sleep makes people happier. Most likely, it's some combination of the two. Insomniacs are also twice as likely to develop depression, and research suggests that treating sleep problems may help successfully treat depressive symptoms.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. A number of studies in adults have found a strong association — though not a cause-effect relationship — between regular sleep loss and the risk of developing diabetes. More sleep may also help reduce diabetes risk for adolescents, according to researchers.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. Other researchers have found that severe sleep deprivation impairs people's ability to follow pre-established procedures for making a go or no-go decision, something that researchers say contributed to the space shuttle Challenger explosion, the Chernobyl meltdown, and the Exxon Valdez disaster.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. In that state, your ability to pay attention is variable at best.Severe sleep deprivation seems to affect your ability to carry on a conversation — much like having 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.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, it said.When people sleep, the body slows down its normal urine production. 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. In adults, it's tied to what's called nocturia — the need to use the bathroom many times during the night.Lack of sleep causes hormonal changes that make it harder for your body to build muscle and heal. This makes it more difficult to recover from muscle damage caused by exercise, and it worsens conditions related to muscle atrophy. Other research has shown that the reverse is also true — during sleep, your body releases growth hormone and heals damage. That's fitness advocates will always point out that sleep is an essential part of getting into shape.People in pain — especially those suffering from chronic pain — tend not to get enough sleep. This makes sense, since 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.Regular sleep loss makes you more likely to develop both Inflammatory Bowel Disease (IBD) and inflammatory bowel syndrome, which affects an estimated 10-15% of people in the US. Patients with Crohn's disease have been shown to be twice as likely to experience a relapse when they don't get enough sleep.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.Our sleep cycle or body clock doesn't just determine when we're tired or awake; it also affects the function of every cell in our body. Researchers have started to figure out how disruptions in sleep schedules prevent cells from fighting inflammation, which could explain why tired people suffer so many problems from inflammatory conditions including asthma, arthritis, multiple sclerosis, and cardiovascular disease.Snoring can be an indication that you are dealing with sleep apnea, a sleep disorder that can cause other medical problems over time. It's caused by decreased airflow, which can strain the heart and cause cardiovascular problems. The condition is also linked to weight gain.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 study participants slept less than six hours per night for a week, researchers found that more than 700 of their 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.Many health problems are associated with sleep deprivation and poor sleep, but here's the big one: People who consistently do not get 7 or 8 hours of sleep per night 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 would otherwise. Lauren Friedman wrote an earlier version of this story.