People on theanine fell asleep a little sooner and rated sleep better
Pooled across 18 trials and 897 people, theanine shortened the time people said they took to fall asleep and improved how they rated their sleep. Every result was self-reported.
What the study found
Across 18 trials, sleep came faster and felt better, modestly
Pooling 18 randomised trials in 897 people, L-theanine shortened how long people said they took to fall asleep, improved how they rated the sleep itself, and left them functioning better the next day. The effects were small to moderate.
The caveat · Self-reported sleep, no recordings
Every outcome here is what people reported about their own sleep; nothing was recorded by a wrist monitor or in a sleep lab. Few of the pooled trials tested theanine on its own rather than mixed with other ingredients, so its individual share stays blurred. And the authors say the best dose and duration are still undetermined.