Serotonin is a neurotransmitter and hormone involved in regulating mood, appetite, sleep, arousal, and pain perception. It is produced mainly in the raphe nuclei of the brainstem and in the gastrointestinal tract, where about 90% of the body's serotonin is found.

Serotonin is synthesised from the amino acid tryptophan. After release into the synapse, it is reabsorbed by the presynaptic neuron through the serotonin transporter. Many antidepressant drugs, including SSRIs such as fluoxetine and sertraline, work by blocking this reuptake, increasing serotonin availability in the synapse.

Functions

• Mood regulation and emotional stability
• Sleep–wake cycles
• Appetite and digestion
• Pain and migraine regulation

Low serotonin activity has been linked to depression, anxiety, and sleep disorders, though the relationship is complex and not fully understood.