Tea and Sleep

Tea can have both positive and negative effects on sleep. True teas, from the Camellia sinensis plant, contain caffeine, a stimulant which can interfere with sleep, especially if consumed close to bedtime. However, tea also contains other compounds that may mitigate these effects.
Herbal teas, however, are a different story. Many herbs have positive effects on sleep and relaxation. Herbal teas are diverse; we discuss them in a separate section at the end of this article.
Caffeine's disruption of sleep
Caffeine is a central nervous system stimulant, and has been shown to interfere with sleep.[1] Caffeine is widely perceived to boost alertness and performance at various tasks, but a review of the scientific literature found that caffeine does not actually boost performance, but merely restores performance degraded by sleep loss.[1]
Consuming caffeine closer to bedtime is especially likely to disrupt sleep. Insomnia and sleep loss can have major impacts on health and ability to function, negatively affecting concentration, creativity, and immune system function. Michael J. Breus, WebMD's expert on sleep medicine, notes that caffeine can stay in your body's systems for up to 12 hours after consumption, and recommends avoiding caffeine within 4-6 hours of bedtime.[2] The Mayo Clinic also recommends to limit your caffeine intake to less than 400 mg per day (5-10 cups of tea or 3-4 cups of coffee) as higher amounts can cause insomnia and other problems.
Different people have varying levels of sensitivity to the effects of caffeine. Some people are able to consume large quantities without any noticeable adverse effects, whereas others may struggle with insomnia even at much lower doses. According to the Mayo Clinic, men are on average more sensitive to caffeine than women. Especially sensitive groups include people taking medications such as Ciprofloxacin (Cipro) or norfloxacin, or certain herbs, including Echinacea.
After only a few days of regular use, the body develops a tolerance to most of caffeine's effects, and thus the most severe sleep disruption is likely to be experienced when people who are unaccustomed to caffeine are exposed to it, or on days when people consume a larger amount than usual. However, population studies have also found negative effects on sleep for people who habitually consume caffeine.[1]