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Serotonin is in a class of chemicals called neurotransmitters, according to the Queensland Brain Institute. These chemicals act as messengers to help neurons communicate with each other. They can also help your nervous system communicate with other parts of the body by sending messages from neurons to muscles. Dopamine is another example of a neurotransmitter.

One of the things that makes serotonin so unique is the number of systems in your body that it plays a role in. According to a study published in Annual Reviews in Medicine, serotonin affects virtually every aspect of mood, and it also plays a role in things like digestion and heart function. Indeed, the researchers point out that despite its clear role in brain function and mood regulation, very little serotonin is actually found in the brain and spinal cord — most of it is located in other areas of the body.

Here we’ll take a look at everything you need to know about this vital neurotransmitter.

Serotonin and your mental health

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Sometimes you just feel happy and content with life. There’s nothing stressing you out, and you can sit back and appreciate the moment, or focus on whatever it is you need to do. When you get into one of those moods, serotonin is likely playing a role. One of the ways that serotonin affects your mood is that it makes you happier and calms you down, according to the Cleveland Clinic. It can also help you pay attention to whatever task is at hand.

The converse is also true: Low levels of serotonin are often found in people with depression and other mental health disorders, according to the Endocrine Society. What is unclear is whether low serotonin levels cause depression, if the opposite is true (via Everyday Health), or if there are other factors altogether working behind the scenes (via BrainFacts.org). In any case, there seems to be a relationship between serotonin levels and mental wellbeing, though the exact nature of this relationship isn’t totally clear.

Some medications elevate serotonin levels

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Because low serotonin levels are associated with mental health conditions such as anxiety and depression, many antidepressant medications target serotonin levels in the body — this class of medication is known as selective serotonin reuptake inhibitors (SSRIs), according to Healthline. They can be used to treat depression, post-traumatic stress disorder, anxiety, and some eating disorders, according to the National Health Service.

SRRI medications work by blocking the reuptake of serotonin into nerve cells. When serotonin travels to nerve cells to deliver messages, they can then be absorbed by the nerve cell. An SSRI prevents that from happening, so that the serotonin can continue delivering messages elsewhere in the body. So, these medications don’t cause your body to make more of the neurotransmitter — they merely make it more available.

Some people can have side effects from SSRIs including loss of sex drive, dizziness, trouble getting or maintaining an erection, and feeling agitated or sick. There are other medications that can be used to treat depression, and there are a variety of SSRI medications, so there are other options available if your medication doesn’t agree with you. When you start taking them, SSRIs can take 2 to 4 weeks before they being working. Typically, your doctor might recommend a 6-month course of the medication after your symptoms improve, though some patients will benefit from taking the medication for a longer period.

Serotonin helps with digestion

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Due to the strong association between serotonin and mood, you may be surprised to learn that most of the serotonin in your body is actually located in your gastrointestinal (GI) tract, says the Cleveland Clinic. Indeed, digestion is one of the primary functions of serotonin. It plays a few roles, such as increasing the rate at which you digest food (an important function if you’ve consumed something a bit off — serotonin helps propel harmful pathogens through your system more quickly) and by aiding in appetite control. However, as a review published in the journal Biochime notes, serotonin also plays a role in inflammation of the gut. People with intestinal problems such as irritable bowel syndrome and ulcerative colitis tend to have overactive serotonin, which increases gut inflammation and causes unpleasant symptoms.

Low levels of serotonin — associated with depression — may also cause digestive issues, according to Columbia University’s Irving Medical Center. Researchers believe this may be because low serotonin can cause slower movement of waste through the digestive tract, leading to constipation.

Workouts boost serotonin levels

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Finding the motivation to hit the gym can be tough if you’re having a bad day, are stressed out, or are just feeling down in general. The effort is often worth the reward you get from your brain, though. Exercise increases your levels of serotonin, according to Harvard Health Publishing. But how does this happen? As it turns out, exercise causes a release of tryptophan in the body — and this amino acid winds up being turned into serotonin in the brain (via Healthline). In turn, this increased serotonin can provide benefits to your mood, making you perk up and feel more at ease (via Cleveland Clinic).

According to Healthline, aerobic exercise is more effective than anaerobic if your goal is to up your body’s content of tryptophan (and therefore, serotonin). Therefore, in order to maximize the serotonin you’ll get from your workout, you might want to consider activities such as running, cycling, or swimming (via Cleveland Clinic).

Serotonin levels can drop in the winter

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People who live in colder, darker climates can suffer from something called seasonal affective disorder (SAD). The drop in mood during winter months could be due to low serotonin levels, according to Harvard Health Publishing. A lack of time spent outdoors due to chilly weather could be the culprit – when people don’t have much exposure to sunlight, their bodies actually produce less serotonin, according to Winchester Hospital. This is reversible, however, and many people find that 45 minutes of light therapy per day is enough to help regulate their mood and fight off SAD. Light therapy is relatively easy to use. You’ll need a specialized light that can be a few feet away from you for 45 minutes. You can put these lights on your desk in your office or near you at home while you’re watching TV or eating. If light therapy doesn’t prove effective for you, you might need medication, such as a selective serotonin reuptake inhibitor (SSRI), to overcome SAD.

Too much serotonin causes serotonin syndrome

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Low levels of serotonin are dangerous and can cause problems like depression and anxiety. It’s also possible to have too much of a good thing. High serotonin levels — a condition known as serotonin syndrome — can also be dangerous. It’s rare for the body to naturally produce too much of the neurotransmitter. However, various medications such as triptans (taken for migraines) and SSRIs can raise your serotonin levels, according to Medline Plus. So, if more than one of these medications is taken at a time, it can raise serotonin levels in the body to unhealthy amounts.

Symptoms of serotonin syndrome include headaches, fast heartbeat and high blood pressure, diarrhea, and abnormal eye movements. If you’re experiencing those symptoms, you should seek treatment. While it can be treated with medication — or by stopping the medication you’re currently taking — serotonin syndrome can be deadly if it’s left untreated. Your doctor should be able to help you coordinate medications to avoid this problem, and many medications that alter serotonin have instructions on the bottle to help you avoid serotonin syndrome.

Serotonin helps regulate breathing

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You breathe hundreds of times per day, and yet you rarely have to stop and think about each breath. What you may be surprised to learn is that serotonin plays a behind-the-scenes role in controlling your breathing, according to the National Institutes of Health. It does this partly by helping to monitor carbon dioxide levels in your body. This was shown in a study performed on mice, where researchers compared the breathing response in mice with normal serotonin levels compared to those whose serotonin-producing neuron function was inhibited (via Science). When exposed to high levels of carbon dioxide, the normal mice breathed faster to expel the harmful carbon dioxide. Mice whose normal serotonin function was blocked didn’t change their breathing much when exposed to more carbon dioxide, suggesting that the communication performed by serotonin in the body is key to regulating our breathing to keep us healthy — and that if your serotonin levels are disturbed, you can have breathing problems.

That said, one study published in the Journal of Clinical Sleep Medicine found that SSRI medications — which help to increase levels of serotonin in the body, per the Mayo Clinic — may actually be associated with breathing problems and lower oxygen levels at night in people who struggle with both depression and sleeping problems. The scientists caution, however, that more research needs to be done to confirm their findings.

Serotonin helps regulate body temperature

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Though you may turn on the air conditioning when it gets hot out, or crank up the heater during the chilly winter months, your body also has its own process to keep itself at a safe temperature: thermoregulation. Through mechanisms like shivering when you’re cold or sweating when you’re hot, your body strives to maintain a core temperature somewhere between about 98 and 100 degrees Fahrenheit, depending on the individual (via Healthline). And as it turns out, serotonin has a role to play in thermoregulation, according to the National Institutes of Health. This was shown in a study on mice who can’t produce serotonin: When researchers exposed these mice to an ambient temperature of 74 degrees F, their body temperatures also dropped to 74 F (via Science). In contrast, mice with normal serotonin function were able to keep their body temperatures regulated, even in cooler outside temperatures.

Serotonin helps the body store and use energy

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The food you eat provides energy for the body, but it doesn’t use all that energy at once. The body stores energy, often in fat cells, for later use. Serotonin plays a role in that process, according to a paper published in Endocrine Reviews. It does this by telling the body to release insulin, which is a hormone that causes the body to store energy. Based on this, the researchers suggest that obesity treatments could focus on lowering the amount of serotonin available in the parts of the body that control metabolism.

In contrast, in the central nervous system — which is the brain and spinal cord — serotonin actually has the opposite effect: It decreases appetite, which makes you consume less energy, and also stimulates brown fat tissue, causing your body to expend more energy (per Endocrine Reviews). So, while serotonin in the central nervous system primes your body to burn energy, serotonin in the metabolic tissues causes it to store energy.

Low serotonin may be connected to obesity

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One of the ways that serotonin decreases the amount of energy you consume is by reducing your appetite, according to a study published in Endocrine Reviews. And according to a paper published in Obesity Reviews, lower amounts of serotonin in the body are linked to obesity. Now, to be clear, the paper’s authors explain that the relationship between serotonin and obesity is multifaceted, and though the two seem to be connected, it’s hard to prove that low serotonin directly causes obesity. However, the authors speculate that serotonin helps to regulate the amount of food people eat, noting that some people have problems with their serotonin levels right before they begin to gain weight. But what makes things even more complicated is that giving people medication to increase their serotonin levels doesn’t necessarily help with obesity, although in some cases it does. It’s a complex matter because other factors play a role, such as an individual’s diet and activity level.

Serotonin plays a role in aggression

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If you find that you’re more irritable than usual, or getting angry at things that normally don’t bother you, there could be a problem with your serotonin. A paper published in Biological Psychiatry explores the role that low serotonin levels play in aggression. The researchers, who were studying changes in impulse aggression, found that giving their test subjects selective serotonin reuptake inhibitors (SSRIs) made them less likely to engage in aggressive behavior.

There are two areas of the brain that the authors note are particularly active when someone is being aggressive: the amygdala and dorsal striatum. They’re not sure whether these areas of the brain create feelings of anger or are merely activated when anger occurs, but they’re definitely associated with aggression. The researchers found, though, that serotonin helps to regulate these areas of the brain, which can reduce aggressive behavior and promote cooperation. They conclude that more study should be done on the effects of serotonin on aggression, in particular the effects of different drugs.