Caffeine is the most widely used psychoactive susbstance in the world. Coffee, tea, mate, soft drinks, cocoa products and candies all contain significant amounts of caffeine.
Table 1. 1995 estimate of caffeine consumption in select countries. Broken down by coffee, tea, mate, and cocoa consumed
Is it safe? Are there side effects?
Caffeine is safe to use both acutely and chronically. However, discontinuing long-term caffeine consumption can cause withdrawal symptoms. These can include marginally increased depression, anxiety, fatigue, and headaches.
In a 1999 review, worldwide caffeine consumption was estimated to be around 70-76 mg/person/day. However, in the US and Canada these levels can reach 210-238 mg/person/day, and in Sweden and Finland it was estimated to be at 400 mg/person/day.
Check out this caffeine calculator at caffeine informer to get an estimate of the amount of caffeine safe to consume based on your weight. See table 2 for an estimate of the caffeine content in common foods and drinks.
Table 2. Caffeine content of select foods and drinks.
Effects on Cognition
Effects of caffeine on Attention and Mood
Caffeine is well-established as an agent to increase alertness. This effect is particularly noticieable in low arousal situations (after benzodiazepine administration, early in the morning, working at night, during illnesses, and in sleep deprivation conditions. Caffeine has also been shown to improve cognitive function in healthy, normal individuals during optimal working conditions.
In a study of 70 participants, those who took caffeine 8 days after withdrawal exhibited improved simple reaction time, and detected targets faster in a cognitive vigilance task. In a study involving 48 participants, it was found that caffeine (250 mg) increased self-rated alertness, but also increased jitteriness and blood pressure. However, the addition of theanine (200 mg) to the mix counteracted the rise in blood pressure. A review published in 2013 showed that caffeine has clear beneficial effects on both simple and complex attention tasks. Furthermore, work has shown that even in the most intense conditions experienced by Navy Seals, a 200 mg dose of caffeine can improve vigilance, reaction time, and alertness, with minimal effects on fine motor functions (as measured by performance on marksmanship).
In terms of negative mood effects, there previously was some concern that caffeine may increase anxiety. However, a 2002 review concluded that “the literature suggests that extremely high doses of caffeine may increase anxiety, but that this is rarely seen within the normal range of ingestive behavior.”
Effects of caffeine on sleep
Large doses of caffeine late in the night will increase the time it takes for some people to fall asleep. The effects of smaller doses varies; however, there isn’t enough evidence to suggest that health is negatively influenced by caffeine-induced sleep disruptions. It was recently shown that long-term evening/nighttime caffeine use delays your circadian rhythm – pushing your internal clock to a later bedtime. Thus, chronic caffeine use can have fundamental effects on your sleep-wake cycle, which should be taken into account when trying to modulate attention and sleep at the same time.
Caffeine & L-theanine
Caffeine and L-theanine can act synergistically to improve alertness and attention. L-theanine can also potentially reverse some of the less favorable physiological effects of caffeine – including increased blood pressure, and reduced flow of oxygenated blood to the head.