It starts when L-theanine increases alpha waves in the brain, which are associated with the meditative or restful state of mind. When these alpha waves increase and the brain relaxes, the activity of the sympathetic nervous system is suppressed (the one that regulates fight or flight responses) and helps to regulate the autonomic nervous system (the one that controls involuntary processes like breathing). When the autonomic nervous system is balanced, the parasympathetic nervous system (the one that relaxes your body after sensing danger) takes over, easing blood vessel constriction and improving blood circulation.

In other words, irritation or anxiety is often due to a disturbance in the autonomic nervous system, which in turn causes poor circulation. L-theanine helps improve blood flow, which can reduce those high-strung feelings. As any student facing an exam can tell you, reducing anxiety and stress is a great way to enhance concentration.

These benefits become self-perpetuating. When you sleep with the parasympathetic nervous system in charge, you’re less likely to stir from your sleep, which in turn leads to a deeper sleep. This deep sleep facilitates smoother recovery from fatigue and is thought to improve the quality of sleep.

This domino effect of relaxation is, according to studies, a great way to balance the less desirable effects of caffeine, such as sleeplessness and overstimulated nerves, which both cause a host of problems for your health.