Before knowing the relationship between serotonin and the sleep cycle, let us know more about serotonin. Serotonin is a neurotransmitter (that also acts as a hormone) and carries signals from the central nervous system (nerves from the brain) to the peripheral nervous system(throughout the body). Serotonin is a natural chemical with the chemical name 5- Hydroxytryptophan found in the brain, bowel, and platelets. The essential amino acid tryptophan constitutes this chemical, thus it is obtained from outside sources. Our brain produces only 10% of serotonin, the rest is found in the lining of our intestine. Most individuals are aware that how well they sleep and how much of it they get can directly affect how well they perform during the day. Some people fall asleep right away, but for others, it is an impossible feat. All night long, they would toss and turn, leaving them feeling worn out and drained. But did you know that your ability to sleep at night might be impacted by what you do during the day? Serotonin is a neuromodulator of sleep. The forebrain and brainstem regions play a key role in regulating and controlling the sleep/wake cycle. Serotonin is an important chemical that plays role in the sleep-wake cycle. Not only it promotes wakefulness but it also inhibits rapid eye movement. Inhibiting REM-sleep effector neurons in the brainstem during waking hours may help serotonin work in conjunction with noradrenaline and acetylcholine to promote cortical attentiveness (inhibitory role on REM sleep). There are at least two reasons for the seeming contradiction between serotonin’s inhibitory and facilitative effects on sleep. The achievement of a behavioral state depends on the intricate interaction between the serotonergic and other neurotransmitter systems. On the one hand, serotonergic modulation of the sleep/wake cycle occurs through a multitude of post-synaptic receptors that mediate different or even opposite responses. REM sleep is induced by the cholinergic neurons in the pons, which are inhibited by brainstem serotonergic and noradrenergic neurons. Lethargy and weariness have been linked to serotonin’s impact on sleep in humans.