Many types of tea (from the Camellia sinensis plant), including green, white, black, and oolong contain L-Theonine.

I’ve been drinking more tea to get more fluid. I noticed that after drinking tea I felt calmer. For a while I assumed it was the whole experience of taking a break, sipping a warm beverage, etc. But it seemed more than that so I did a bit of research.

Tea contains L-theanine, an amino acid that has been shown to have an anti-anxiety effect. It increases dopamine and serotonin levels in the brain and increases production of alpha waves, which are more predominant in a resting state.

Psychotropic Effects Of L-Theanine And Its Clinical Properties: From The Management Of Anxiety And Stress To A Potential Use In Schizophrenia, Pharmacological Research, September 2019

Anxiety disorders are highly prevalent in modern societies, and are ranked the sixth most important contributor of non-fatal negative health outcomes. L-theanine is an amino acid naturally found in green tea (Camellia sinensis) and some other plant extracts, and recent clinical studies have proposed promising adjuvant effects of L-theanine for the negative impact of anxiety and psychological stress* on health.

* Psychological stress can be defined as acute, for example in anticipation to a surgical procedure, or chronic, for example in financial or family difficulties. Both forms of stress have the potential to disrupt the nervous, endocrine and immune systems.

Anxiety and stress outcomes

Overall, several studies have shown that the administration of L-theanine improved anxiety and stress outcomes, alongside improvements in other manifestations such as depression and psychopathological symptoms.

Additionally, verbal memory and executive function were improved.

Most of the studies appraised in our review recruited from 12 to 60 participants, were double-blinded and tested the effects of L-theanine in doses ranging from 15 to 400 mg.

Blood pressure lowering effects

L-theanine may have the potential to lower blood pressure, possibly indirectly via reduction of the manifestations associated with stress, inhibiting cortical neural excitation and consequently attenuating sympathetic activity.

Major depressive disorder (MDD)

Hidese and colleagues found that L-theanine administration reduced depressive symptoms and improved cognitive function in MDD patients.

Natural sleep aid

Overall, it is suggested that the intake of 200 mg of L-theanine at bedtime may improve sleep quality by anxiolysis [decreasing anxiety] rather than sedation.

Mechanism

Structurally, L-theanine is a glutamate analogue, hence binding to the same glutamate receptors and therefore hindering the neuroexcitatory effects triggered by glutamatergic activation. It is believed that L-theanine mediation on glutamatergic neurotransmission is the main pathway by which this non-proteinaceous amino acid is able to attenuate anxiety disorders and mitigate the negative outcomes of exposure to acute and chronic stress.

Furthermore, similarly to GABA administration, L-theanine was able to significantly increase the generation of alpha waves, which are more predominant in adults at rest with eyes closed, but also in relaxed conditions. … Studies employing animal models have found that the administration of L-theanine seems to increase GABA* levels in the brain.

[Studies in animals] given L-theonine have found higher levels of dopamine and serotonin.

*GABA: Gamma-aminobutyric acid, the chief inhibitory neurotransmitter in the brain.

Some other good reading on the topic:
The Effects Of Green Tea Amino Acid L-Theanine Consumption On The Ability To Manage Stress And Anxiety Levels: A Systematic Review, Plant Foods for Human Nutrition, November 2019

The paper below found that black tea had more L-Theanine than green but other studies found the opposite. It’s about ten years old so the tea we’re buying today is a different animal than the tea they tested. It did find, however, that adding milk lowered the L-Theanine content. (Most of the L-Theanine is extracted within the first 5 minutes of brewing.)
How Much Theanine In A Cup Of Tea? Effects Of Tea Type And Method Of Preparation, Food Chemistry, 2011

From the Introduction:

L-Theanine is a non-proteinogenic amino acid found in tea (its primary dietary source) and a derivative of the amino acid glutamic acid. Tea is the most widely consumed beverage worldwide after water and is generally seen as a relaxing drink, whereas coffee is seen as more of an energising drink. Although both drinks contain caffeine, L-theanine is only present in tea. This difference has led to an interest in the effects of L-theanine on psychological measures, such as stress and anxiety. L-Theanine has been found to have a relaxing effect and to increase alpha brain waves, synonymous with a relaxed yet alert state. Recent research has found that L-theanine reduced both subjective and physiological stress responses during a stressful task situation and that it may be useful for reducing raised blood pressure. This relaxation effect of L-theanine may also explain the improvements seen in subjective sleep quality and mood upon awakening after L-theanine.

So, the relaxing effect may have been from the tea itself, in addition to the tea-drinking experience.