Green tea is the unfermented product of the Camellia sinensis plant. Part of green tea’s appeal is how it makes you feel -- alert, relaxed and energetic. The health benefits green tea have been documented in extensive testing of its antioxidant, cholesterol--lowering, anti--allergen and antihistamine properties. Green tea’s effect on the central nervous system depends on how much you consume and in what form.

Active Ingredients

Green tea is a stimulant. All tea contains caffeine and smaller amounts of theobromine and theophylline. Theobromine is a smooth muscle stimulant. Theophylline, a smooth muscle relaxant, causes restricted air passages to open and is used to treat asthma as it makes breathing easier. Neither is as powerful as caffeine which stimulates the entire central nervous system.

Caffeine

The strongest stimulant in green tea is caffeine, the most widely consumed psychoactive medication in the United States, according to the University of Utah College of Pharmacy. Caffeine’s effects on the nervous system include increases in alertness, focus, feelings of well--being, good moods, quick thinking and ability to engage longer in an intellectual activity. In extreme cases, over--consumption of caffeine can cause nervous twitching, hallucinations and anxiety.

Potent Relaxant

Green tea contains L-theanine, an amino acid that raises levels of dopamine and serotonin, according to Memorial Sloan--Kettering Cancer Center. Theanine is considered a relaxant that calms the brain and the entire nervous system without any reported side affects or drowsiness. When combined with caffeine and other stimulants, as it is in green tea, theanine promotes a feeling of ease and alertness. In tests published in “Nutritional Neuroscience” in December 2010, researchers found that a combination of theanine and caffeine reduced tiredness in test subjects and heightened their accuracy in performing demanding cognitive tasks.

Excess Consumption

Drinking too much green tea or taking high doses of extracts can overstimulate the nervous system and lead to undesirable effects. Excessive caffeination can cause tremors, dizziness, confusion, insomnia, restlessness, agitation and physical symptoms such as irregular heartbeat. According to the Cam--Cancer Center, it would take amounts of green tea far in excess of normal consumption to produce extreme negative effects. Cam--Cancer recommends that concentrated extracts of green tea be accompanied by educational brochures, detailing proper doses. When consumed in safe doses, and not on an empty stomach, green tea extracts do not result in overstimulation from caffeine.