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L-theanine is an amino acid naturally found in tea leaves. First linked to tea’s calming effects in the 1940s, it is now sold as a supplement for stress relief, focus and better sleep. Research into its effects has so far been limited, but there are intriguing hints it may have several benefits for the brain, particularly in people who have anxiety.

What beneficial effects might L-theanine have?

Research into L-theanine is promising, but still in its infancy. “The research is limited, the studies are small and the results are mixed,” says at Northumbria University in the UK.

However, data from small trials suggest L-theanine can lower a person’s while they are doing a task that requires attention, and, during times of stress, it may help lower a person’s and temper spikes of . There is some evidence, too, that it might boost relaxation and focus.

What effects does L-theanine have on the brain?

Once absorbed into the bloodstream via the gut, L-theanine is able to. It has a similar structure to the brain chemicals glutamate and GABA, which are neurotransmitters involved in turning signaling up and down across the brain. Studies in rodents also suggest L-theanine may boost levels of , which are involved in many aspects of brain functioning, including mood and reward.

L-theanine also seems to , which is linked to a state of calm, alert wakefulness. “If you close your eyes, alpha goes up all of a sudden,” says at the University of Rochester in New York.

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Is this beneficial for mood and well-being?

In general, an increase in alpha brain waves might help the brain tune out distractions and focus, and L-theanine may enhance this effect. “The higher the alpha, the more you’re tuning out the world,” Gomez-Ramirez explains. This effect is stronger when combined with caffeine, as the two compounds “have some sort of synergistic effect”, he says.

The effect of L-theanine on alpha waves seems more pronounced in , and a study in stressed rats found L-theanine’s effect on was linked with fewer depression-like symptoms. However, evidence in humans is inconsistent, with some studies finding no benefit for and others .

If it does ease anxiety, this might explain L-theanine’s purported benefits for sleep. found it helps people fall asleep faster and improves sleep quality.

Is there enough L-theanine in tea to have an effect?

Tea – including black teas and green teas such as matcha – contains L-theanine at varying levels, around 5 to 40mg per cup. It is possible this might offer mild effects, but most L-theanine studies use doses around 200 mg, similar to the level typically found in supplements.

How good is the evidence on L-theanine?

Most studies use small sample sizes and track only short-term effects, often testing a single dose or a few weeks of supplementation. “It’s certainly promising, but we just need more work in this area” says Amanda Bulman at the University of Canberra in Australia.

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One complication is studies often combine L-theanine with other ingredients, which clouds the evidence. “It’s hard to tell what’s actually doing the work,” says Haskell-Ramsay. “It’s impossible to pick out which of these things is having an effect.”

Haskell-Ramsay also warns, “people are often taking it daily, long term and we just don’t have the data on that”.