A chemical in the brain and body that helps control mood, sleep, appetite, and other functions. It is often linked to feelings of well-being and happiness.
From 'sero-' based on 'serum' (the clear part of blood) + 'tonin' from 'tonic', because it was first found in blood serum that affected blood vessel tone. It was named in the mid-20th century as scientists studied neurotransmitters.
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