“Overnight tea” refers to tea that has been brewed traditionally with hot water, then left with the tea leaves steeping in the water for an extended period—typically overnight.

Due to the high temperature and prolonged steeping, compounds like tannins and caffeine continue to be released into the tea, resulting in a brew that is noticeably more bitter and astringent, and may cause heart palpitations or digestive discomfort. If not stored in a cool environment, the tea becomes more susceptible to bacterial and microbial contamination from the air, which is why older generations often warned that overnight tea should not be consumed.