The current evidence shows that supplementation with L-theanine may help reduce stress and anxiety in people experiencing acutely stressful situations.[1] Furthermore, L-theanine may also prevent the increase in blood pressure caused by stressful mental tasks.[2] Some small studies have also found that L-theanine might lower depression and anxiety scores in healthy people and in people with major depressive disorder.[3][4]
Supplementation with L-theanine may also improve some aspects of cognitive function, including attention, executive function, and memory.[5][6][3][4] That said, L-theanine has been found to impair executive function and lower attentional control when people are in an emotionally aroused state[7] or are undertaking stressful mental tasks.[8]
Further evidence suggests that supplementation with L-theanine might also enhance sleep quality[3][4] by promoting a more relaxed state in the brain.[9] However, it is important to note that this evidence is derived predominantly from studies of healthy people. There is currently insufficient evidence to make firm conclusions about whether theanine can improve stress, anxiety, depression, cognitive function, or sleep quality in people with chronic conditions.