The Federal Aviation Administration’s employees are extremely tired, according to the Associated Press (AP).

The AP got its hands on the findings of an unpublished study that found that in 2011, air traffic controllers suffered from such fatigue — a lack of energy and motivation, in response to stress, boredom, and lack of sleep — that it impaired their judgement at work. Some controllers, for example, jeopardized flight navigation by allowing planes to fly too close together. The AP also cited that 60 percent of air traffic controllers fell asleep while driving to their late shifts, while 20 percent reported committing “significant errors” due to lack of sleep.

Working the night shift, as air traffic controllers typically do, is a known cause of fatigue. Not only does fatigue increase risk of errors at work, but studies show working an irregular schedule can also increase risk of cardiovascular disease and mortality. Additional research even shows that tiredness can increase risk of some cancers, diabetes, and infertility. Even so, everyone experiences fatigue differently.

“Fatigue is personal and individualized,” Dr. Adam Rindfleisch, of the University of Wisconsin's Department of Family Medicine, Integrative Medicine, told The Daily Burn. “Since there are a number of reasons why someone could feel fatigued, it’s important your doctor tailors his or her diagnosis to your individual symptoms and needs.”

Since the air control study specifically mentions sleep, we’ll break down the effects poor sleep-induced fatigued has on some parts of the body.

Brain

Research continues to show poor sleep habits worsen health. It’s not just the amount of sleep people get that’s important, it’s the quality of sleep they get, too. Chronic sleep deprivation has been linked to cardiovascular disease, depression, and even brain damage, Mitchell Moffitt and Greg Brown wrote in their book AsapSCIENCE: Answers to the World’s Weirdest Questions, Most Persistent Rumors, and Unexplained Phenomena.

“After your first sleepless night your mesolimbic system becomes stimulated and dopamine runs rampant. And this may actually trigger some energy, motivation, positivity, even sex drive,” Moffitt and Brown said. “Sounds appealing, but it’s a slippery slope! Your brain slowly begins to shut off the regions responsible for planning and evaluating decisions, leading to more impulsive behavior. Once exhaustion sets in, you’ll find yourself with slow reaction times and reduced perceptual and cognitive functions.”

Muscles

Fatigue is marked by muscle weakness, a symptom that can exacerbate existing conditions, like arthritis. According to a 2008 study published in The Journal of Physiology, one of the many mechanisms responsible for weakness and decreased exertion is “the accumulation of metabolites within muscle fibers.” However, researchers note these mechanisms are specific to the tasks being performed.

“Muscle fatigue, it seems, can refer to a motor deficit, a perception or a decline in mental function; it can describe the gradual decrease in the force capacity of muscle or the endpoint of a sustained activity; and it can be measured as a reduction in muscle force, a change in electromyographic activity, or an exhaustion of contractile function,” the researchers wrote.

Skin

Yep, fatigue can literally show on your face. Moffitt and Brown cited studies that have shown a direct correlation between sleep deprivation and a person’s perceived beauty. “That is to say, sleep-deprived individuals appeared less healthy and less attractive than when they were well rested,” the two concluded.

Everyday Health reported, too, that not enough sleep exacerbates existing skin conditions. It increases “inflammatory response,” which is why fatigued patients may experience more breakouts, sensitivity, and dullness. In fact, with an increased amount of inflammatory cells in the body, it causes the body to breakdown more collagen and hyaluronic acid, the two molecules responsible for your skin’s glow.

Correction: This article originally conflated chronic fatigue and chronic fatigue syndrome. Chronic fatigue syndrome is a neurological disorder, not a result of poor sleep.