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Contrary to popular belief, we don’t have a limited supply of brain cells. Sure, the majority of them grow early in life, but some areas of the brain continue to grow them into adulthood and beyond — a process called neurogenesis. The hippocampus is one of these areas, and it’s also among the most important areas of the brain, playing roles in memory, emotion, and learning. But as we grow older, a number of factors compete with neurogenesis and kill brain cells. No, they’re not smoking weed and drinking alcohol; instead, they’re a little more common than you might expect. According to Medical Daily, here are seven of them.  

Losing Sleep

The National Sleep Foundation recommends most adults get between seven and nine hours of sleep every night, and for good reason. This gives our brains sufficient time to move through the sleep stages, which gradually become deeper, and end in REM (rapid eye movement) sleep before starting all over again.

Getting to REM sleep is important because it’s during this time that the brain is working hard both to keep us asleep — by causing paralysis in limbs during dream states, for example — while also activating brain regions responsible for learning. This is when memories are consolidated and stored, and energy levels are replenished. With that said, it’s not surprising that someone who consistently loses sleep will have a harder time concentrating, making decisions, and engaging themselves in both learning and social situations.

A study from last year shows how these effects fall in line with brain damage; it found the neurons in the energy-producing region of the brain called the locus coeruleus (LC) began to die from extended wakefulness. Without these cells to produce energy, our bodies are unable to function properly the next day. Another study found sleep deprivation could cause shrinkage in the cerebral cortex and hippocampus, especially in adults over 60 years old, suggesting that sleep becomes ever more important as we age.

Smoking

Over 42 million adults smoke cigarettes in the United States; that’s nearly one in five people. And with each drag, they’re inhaling over 7,000 toxic chemicals, 69 of which have been shown to cause cancer. Smoking causes a whole range of other diseases, too, from chronic bronchitis to emphysema, to heart disease and stroke.

Stroke, in itself, has been shown to cause brain damage, but with all these toxins, there’s bound to be at least one that specifically affects the brain, right? It turns out there are several of them. In a 2002 study from France’s National Institute for Health and Medical Research, scientists found nicotine-addicted rats generated 50 percent fewer neurons in the hippocampus’s dentate gyrus. Those that took higher doses of nicotine also experienced the most brain cell death. If that isn’t bad enough, another study from India found a compound in cigarettes, called NNK, could cause an exaggerated response in the brain’s white blood cells, forcing them to attack healthy brain cells as well.

Researchers from the rat study believe their findings could explain why smokers who try to quit experience short-term cognitive problems. “It could be that while they are smoking, the stimulant effect of nicotine masks the loss of neuronal plasticity,” study author Pier Piazza told New Scientist. “When they stop smoking, these deficiencies remain.”

Dehydration

The common misconception that alcohol will kill brain cells comes from the fact that it triggers a whole range of body processes. One of these is “breaking the seal”; that moment when we have to use the bathroom after downing a couple of beers, and revisiting it more often than usual for the rest of the night. As we drink more, the alcohol suppresses the hormone vasopressin, which is responsible for retaining water in the body. This makes our bodies unable to hold our urine in, and one unfortunate result is dehydration — which also happens to be the reason we get hangovers.

Considering 75 percent of the brain is made of water, it would behoove anyone drinking to couple their vodka-cranberry with a large glass of water. Actually, we’re going to recommend that glass of water being around at all times of the day, because mild dehydration can occur after only four hours. Once it starts getting bad, it can cause the brain to swell (cerebral edema) as the body tries to pull more water into the cells — some of these may eventually rupture. Seizures aren’t uncommon either, as electrolyte imbalances cause neurons to miscommunicate. All of this causes the brain to work harder to function properly, which in turn can cause it to shrink, a 2011 study found.

Stress

Most of us can agree there’s been that one day or week where everything seemed to be going wrong. Trying to juggle so many problems is stressful. But because it’s the body’s way of rising to the challenge, it often helps us with improved concentration and focus. However, the body can only take so much of it, and after a certain point, it’ll backfire, throwing off concentration, causing irritability, and killing our energy.

All of this can be blamed on the stress hormone cortisol, which is released from the adrenal gland during stressful moments. The hormone activates several biological processes with the intention of diverting energy to where it’s most needed; digestion, for example, stops while heart rate increases.

In people with chronic stress, however, cortisol levels can be so excessive that the brain ends up producing more myelin-producing cells and fewer neurons, a study from last year found. Myelin is the fatty material that makes up the brain’s white matter and quickens communication between neurons. But such changes in the brain, the researchers said, may contribute to a person’s risk of mental health illnesses like schizophrenia and anxiety disorder.

Cocaine and Other Narcotics

While marijuana won’t kill brain cells, other narcotics on the Drug Enforcement Administration’s drug schedule list certainly will. Drugs like cocaine, methamphetamine, bath salts (synthetic cathinones), and MDMA (ecstasy or molly) all activate the brain’s reward systems, triggering the release of the neurotransmitters serotonin, dopamine, and norepinephrine.

While triggering all these neurotransmitters can cause a euphoric high and make the user especially active, they also damage the neurons responsible for releasing the feel-good chemicals. As a result, the user can develop a tolerance to the drug, which forces them to take more over time to achieve the same high — further damaging the cells or even killing them. In a 2003 study on cocaine, researchers looked at brain samples from 35 deceased cocaine users and compared them to the brains of 35 nonusers. They found dopamine levels were far lower in those who had used cocaine, especially in those who were depressed as well, suggesting that there were fewer dopamine-producing neurons.

Researchers believe that this damage to brain cells is what leads to drug addiction, as the user finds it progressively harder to feel good on his own.