L-theanine is an amino acid present almost exclusively in the tea plant (Camellia sinensis).
L-theanine increases dopamine and serotonin production in the brain.
It also increases alpha-brain wave activity, a sign of induced relaxation.
A small Japanese study of university students showed oral L-theanine administration of 200 mg led to increased alpha-brain waves and a subjective sense of relaxation.
Theanine administration caused a relaxed, yet alert, state of mind without sedation, beginning approximately 40 minutes after oral dosing.
Green tea is often used as a relaxing beverage, but I don’t recommand drinking green tes as it may contain as much harmful and addictive caffeine as coffee. Theanine seems to counteract the stimulant effect of caffeine to some degree.