A recent study published in the British Journal of Nutrition provides evidence that a high-dose combination of L-theanine and caffeine can improve attention and reaction time in people who are acutely sleep-deprived. The researchers found that participants who took this combination were better at identifying potential road traffic accidents in a computerized task and responded more quickly than when given a placebo. Brainwave data also indicated stronger and faster neural responses tied to attention.
L-theanine is a naturally occurring amino acid found in tea leaves, while caffeine is a stimulant found in coffee, tea, and cocoa. Both substances are commonly consumed worldwide and are available in purified form as supplements. There has been growing interest in their potential to support cognitive functions such as attention, especially when combined.
Researchers have studied L-theanine and caffeine separately and together in previous work. Caffeine is well known to improve alertness and reaction time, while L-theanine is thought to have calming effects and may support focus without causing drowsiness. When taken together, some studies suggest they might work in a complementary way, possibly enhancing attention more than either compound on its own.
People often consume caffeine and L-theanine during times of fatigue, such as after a poor night’s sleep. Despite this, earlier research has mostly focused on well-rested participants. The new study aimed to test whether a high dose of L-theanine and caffeine could improve attention in individuals who were deliberately kept awake to simulate real-world fatigue.
“Sleep deprivation is increasingly common among young adults due to academic, professional, and lifestyle demands, yet it impairs attention, a key function for safe driving and daily performance,” explained study author Gayani Nawarathna, a lecturer at the University of Peradeniya and MPhil candidate in cognitive neurophysiology at the University of Peradeniya.
“People often tend to use theanine or caffeine when they feel tired or sleep-deprived. While both compounds are known to improve alertness, their combined effects on attention and brain function under sleep-deprived conditions were not well understood. We aimed to determine whether this natural combination could effectively improve the attention of young adults who had experienced a short period of sleep loss (17 hours).”
The researchers conducted a double-blind, placebo-controlled trial with 37 healthy young adults between the ages of 22 and 30. All participants were kept awake for 17 hours before the experiment, ensuring they were in a state of acute sleep deprivation. On separate days, each person received either a capsule containing a combination of 200 milligrams of L-theanine and 160 milligrams of caffeine or a placebo.
First, they completed a traffic-related computerized task. Then the capsule was given. About 50 minutes after taking the capsule, participants completed the same task again. They had to press a button as quickly as possible whenever they saw an image that suggested an imminent traffic accident, while ignoring more frequent images of safe driving situations. Brain activity was measured at the same time using EEG, a technique that records electrical signals from the scalp.
The results showed that the L-theanine–caffeine combination significantly improved both accuracy and reaction time. Participants were better at correctly identifying imminent accidents and responded about 40 milliseconds faster, on average, compared to when they took the placebo. This difference in reaction time may seem small, but in real-world driving, even a brief improvement can make a difference in avoiding accidents.
Importantly, brainwave data supported these behavioral improvements. The researchers focused on a specific brain signal known as the P3b component, which is linked to attention and decision-making. After participants took the active treatment, this brain signal was larger and occurred more quickly. These changes suggest that the brain was allocating more resources and processing information faster during the task.
“Our results suggest that a high dose of L-theanine (200 mg) combined with caffeine (160 mg) can enhance attention when a person is acutely sleep-deprived,” Nawarathna told PsyPost. “Participants who received the combination responded faster and more accurately to potential accident scenes in a computerized traffic-related task. On average, their reaction times improved by about 40 milliseconds more than with a placebo. The brain recordings showed stronger and faster attentional processing.”
While the findings point to a potential benefit of taking L-theanine and caffeine together during periods of sleep deprivation, there are some limitations to consider. The participants were tested in a controlled lab setting using a computer task. Whether these benefits would carry over to real driving situations remains to be seen.
“Our findings should be interpreted with some caution. Although we used a traffic-related task to simulate real-world settings, actual driving involves much more complex decision-making processes. Also, our study examined the effects of L-theanine-caffeine combination after just one night without sleep, not long-term sleep deprivation. People who are regularly sleep-deprived or who frequently consume large amounts of caffeine or L-theanine may not experience the same benefits, since the body can build up tolerance to these compounds over time.”
The researchers suggest that future studies could explore the effects of L-theanine and caffeine in driving simulators or on-the-road tests. It would also be useful to understand how different doses and long-term use might influence attention, especially in people who consume caffeine regularly.
The study, “High-dose L-theanine–caffeine combination improves neurobehavioural and neurophysiological measures of selective attention in acutely sleep-deprived young adults: a double-blind, placebo-controlled crossover study,” was authored by Gayani S. Nawarathna, Dewasmika I. Ariyasinghe, and Tharaka L. Dassanayake.
