Serotonin is known as the "happy hormone" because it contributes to a feeling of happiness and well-being, while it acts as a hormone and a neurotransmitter. 95% of the body's serotonin is found in the gut, though it's also in the brain, blood platelets, and central nervous system. Serotonin can't cross the blood-brain barrier, so any serotonin needed by the brain must be produced by the brain. If the brain has too little serotonin, it may cause depression. If it has too much, it can lead to excessive nerve cell activity.

Serotonin's scientific name is 5-hydroxytryptamine (5-HT), and it has several important functions:

• Controls sleep and wake cycles.
• Regulates mood; low serotonin levels are associated with depression.
• Plays a role in memory and cognition.
• Regulates anxiety.
• Controls bowel function.
• Is critical to bone health.
• Heals wounds.
• May have a role in obesity, migraines, and Parkinson's disease.
• Low levels are associated with eating disorders, obsessive-compulsive disorder, panic disorder, post-traumatic stress disorder, and social anxiety disorder.
• Gets rid of foods that don't agree with you through nausea and diarrhea.
• Low levels are associated with increased libido.
• Is associated with breast milk production, liver regeneration, and cell reproduction.

Depression is a complex topic and narrowing the causes of depression to one factor isn't realistic. However, research has shown that some people who deal with depression have reduced serotonin transmission, and low levels of a serotonin byproduct have been linked to a higher rate of suicide.

Serotonin doesn't necessarily cause depression, but increasing the amount of serotonin available in the body through selective serotonin reuptake inhibitors (SSRIs) is one of the most common and effective treatments for depression. According to research from Germany, 40% to 60% of people with moderate to severe depression show improvement after 6 to 8 weeks of SSRI treatment. The SSRIs are believed to increase positive emotional processing, which improves mood. SSRIs are anti-inflammatory, and inflammation has been associated with depression.

The first observations of the biochemical events of sleep were made over 40 years ago. It's now clear that several neurotransmitters, neuropeptides, and neurohormones are involved in the sleep/wake cycle. According to Progress in Neurobiology, serotonin has been known to play an important role in sleep, but how and where is controversial.

What we do know is that vitamin D3 promotes the production of serotonin. At night, serotonin is synthesized by the pineal gland to make melatonin, the sleep hormone. If you're not getting enough serotonin during the day, you may have trouble sleeping at night. This is why experts recommend getting 10 to 30 minutes of sunlight on your skin per day to promote your production of serotonin.

Serotonin levels are lower in sleep than when awake, and are lowest during rapid-eye-movement sleep (REM), or when dreaming occurs. This is why SSRIs reduce REM or dreaming sleep.

There are several signs of serotonin deficiency. Some physical signs are:

• poor memory
• cravings for sweets and starchy foods
• insomnia
• aggression
• low self-esteem
• fatigue
• weight gain
• nausea
• constipation or other more serious bowel problems such as irritable bowel syndrome

There are other psychological issues that are associated with low serotonin levels:

• eating disorders
• obsessive-compulsive disorder
• panic disorder
• post-traumatic stress disorder
• social anxiety disorder

According to the University of California San Francisco Children's Hospital, serotonin levels in your blood should be between 101 to 283 nanograms per milliliter (ng/ml). Normal ranges may vary slightly between laboratories; your doctor will interpret your blood test results.

Higher serotonin levels may indicate carcinoid syndrome. Carcinoid tumors are rare and develop from hormone-producing cells called enterochromaffin cells that occur throughout the body, with 65% starting in the gut and 25% in the lungs.

SSRIs are selective serotonin reuptake inhibitors, and are the most commonly prescribed antidepressants, though they are also used for anxiety disorders. They ease the symptoms of moderate to severe depression and are generally considered safe, causing fewer side effects than other antidepressants.

They work by increasing serotonin levels in the brain. SSRIs prevent the reabsorption (reuptake) of serotonin into neurons, making more available in the brain, improving the transmission of messages between neurons (thus a neurotransmitter).

The Food and Drug Administration (FDA) has approved the following SSRIs to treat depression:

• Zoloft (Sertraline)
• Prozac (Fluoxetine)
• Lexapro (Escitalopram)
• Paxil (Paroxetine)
• Celexa (Citalopram)

Some people experience no side effects with SSRIs, and for some, the side effects go away after a few weeks. Possible side effects include:

• headache
• dry mouth
• nausea, vomiting, and/or diarrhea
• insomnia
• nervousness
• drowsiness
• sexual issues
• dizziness
• increased or decreased appetite

Excessive levels of serotonin can cause serotonin syndrome. This is usually caused by taking more than one medication, such as an antidepressant with a migraine medication. It can also be caused by an overdose of antidepressant medications. Drugs that can cause serotonin syndrome include:

• SSRIS
• serotonin and norepinephrine reuptake inhibitors (SNRIs)
• bupropion
• tricyclic antidepressants
• monoamine oxidase inhibitors (MAOIs)
• migraine medications
• pain medication
• lithium
• some herbs, such as St. John's Wort
• some over-the-counter cough and cold medications
• cocaine, LSD, ecstasy, and amphetamines

Cocaine, LSD, ecstasy, and amphetamines can raise your serotonin levels by blocking the transport of serotonin, dopamine, and norepinephrine. According to the National Institute on Drug Abuse (NIDA), sharp spikes in dopamine (the "happy hormone" that's part of your brain's reward system) in the nucleus accumbens (NAc) create the addictive effect, along with extracellular serotonin in the ventral pallidum (VP) and orbitofrontal cortex (OFC). Researchers found that cocaine-induced increases in serotonin in the VP can weaken one's ability to curb impulsive behaviors.

A study published in Neurology examined the brain of a 26-year old man who started using ecstasy (whose chemical name is methylenedioxymethamphetamine, or MDMA) once a month at age 17. MDMA causes neurons to release serotonin and causes an increased awareness of emotions and a heightened sense of intimacy. He continued using ecstasy and in the last 3 years of his life, he used it 4 to 5 nights per week at "rave" clubs, often with a 3-day weekend binge when he took 6 to 8 tablets. His friends reported that after these binges he appeared depressed and had slow speech.

In the last months of his life the man began using heroin and cocaine. His brain was compared to the autopsies of 11 normal individuals. The levels of serotonin and other chemicals associated with serotonin were 50% to 80% lower in the brain of the MDMA user. Researchers believe that the behavioral effects of the drug as well as the depression afterward are probably due to the massive release and depletion of serotonin. The lead researcher said that research should be done to determine whether increasing levels of serotonin in people who are going off of the drug would help eliminate some of the behavioral problems that occur during withdrawal.