Serotonin definition: a monoamine neurotransmitter that’s functionally important in neural transmission for regulating the sleep/wake cycle, depression and memory. Serotonin foods high in the amino acid tryptophan can maintain healthy serotonin levels, but lifestyle choices like traveling abroad or working a split shift can cause erratic sleep schedules that will disrupt serotonin production. When serotonin levels are low, the inability to sleep easy and other health issues can result, including depression and chronic fatigue syndrome. Read more »