There are a variety of chemicals in our body called neurotransmitters, which transmit signals from one nerve cell (neuron) to another. This "neuro" part of the word is also where "neurotic" comes from, which used to be a common label for mental illness, but is no longer the preferred term.
The two most relevant neurotransmitters for mental health are serotonin and dopamine.
Serotonin is associated with feelings of well-being and happiness, but it also serves to control bowel movements (90% of serotonin is found in your GI tract!), help form blood clots, regulate sleep, and is related to bone health, learning, and memory. Common antidepressant medications increase your levels of serotonin by blocking the absorption of serotonin by nerve cells, keeping more of it available for passing along further messages between nerve cells in the brain. Meditation, bright lights (such as the lamps intended for use related to seasonal depression), and exercise are also known to increase serotonin levels.
Dopamine’s role in pleasure and reward is that it helps your brain to recognize “incentive salience.” This means that it’s like a little red flag to your brain, saying “hey, pay attention, this is about to feel good, and you want to remember this, so you can do it again.” When this incentive salience becomes dysregulated, it can manifest as addiction. Dopamine also plays a role in Parkinson's Disease, ADHD, schizophrenia, and bipolar disorder. There's a lot to unpack, so I recommend reading this article from Psychology Today, which goes into even more detail.
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