Caffeine is the most widely used psychoactive substance in the world.
Tea, coffee, mate, soft drinks, cocoa products and candies all contain significant amount of caffeine.
Effects on Cognition
Caffeine is well-established as an agent to increase alertness. In particular, caffeine has been shown to increase alertness in low arousal situations (after benzodiazepine administration, early in the morning, working at night, during illnesses, and in sleep deprivation conditions. But caffeine has also been shown to improve cognitive function in healthy, normal individuals during optimal working conditions as well.
In a study of 70 participants, those who took caffeine 8 days after withdrawal exhibited improve simple reaction time, and detect 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 led to a counteract 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 dose of 200 mg of caffeine can improve vigilance, reaction time, alertness, with minimal effects on fine motor functions (as measured by performance on marksmanship).
In terms of negative mood affects, there previously was some concern in the field that caffeine may increase anxiety. However, in a 2002 review the author 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 go 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. Recently, however, it was shown that long-term, later night, caffeine use delays your circadian rhythm - pushing your internal, biological 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.
- Fredholm, B. B., Battig, K., Holmen, J., Nehlig, A., & Zvartau, E. E. (1999). Actions of caffeine in the brain with special reference to factors that contribute to its widespread use. Pharmacol Rev, 51(1), 83-133. Retrieved from http://pharmrev.aspetjournals.org/content/51/1/83.full.pdf
- Silverman, K., Evans, S. M., Strain, E. C., & Griffiths, R. R. (1992). Withdrawal syndrome after the double-blind cessation of caffeine consumption. N Engl J Med, 327(16), 1109-1114. doi:10.1056/nejm199210153271601
- Johnson, L. C., Spinweber, C. L., & Gomez, S. A. (1990). Benzodiazepines and caffeine: effect on daytime sleepiness, performance, and mood. Psychopharmacology (Berl), 101(2), 160-167.
- Smith, A., Kendrick, A., Maben, A., & Salmon, J. (1994). Effects of breakfast and caffeine on cognitive performance, mood and cardiovascular functioning. Appetite, 22(1), 39-55. doi:10.1006/appe.1994.1004
- Smith, A. P., Kendrick, A. M., & Maben, A. L. (1992). Effects of breakfast and caffeine on performance and mood in the late morning and after lunch. Neuropsychobiology, 26(4), 198-204. doi:118920
- Smith, B. D., Davidson, R. A., & Green, R. L. (1993). Effects of caffeine and gender on physiology and performance: further tests of a biobehavioral model. Physiol Behav, 54(3), 415-422.
- Leathwood, P. D., & Pollet, P. (1982). Diet-induced mood changes in normal populations. J Psychiatr Res, 17(2), 147-154.
- Warburton, D. M. (1995). Effects of caffeine on cognition and mood without caffeine abstinence. Psychopharmacology (Berl), 119(1), 66-70.
- Smith, A. P., Christopher, G., & Sutherland, D. (2013). Acute effects of caffeine on attention: a comparison of non-consumers and withdrawn consumers. J Psychopharmacol, 27(1), 77-83. doi:10.1177/0269881112460112
- Rogers, P. J., Smith, J. E., Heatherley, S. V., & Pleydell-Pearce, C. W. (2008). Time for tea: mood, blood pressure and cognitive performance effects of caffeine and theanine administered alone and together. Psychopharmacology (Berl), 195(4), 569-577. doi:10.1007/s00213-007-0938-1
- Einother, S. J., & Giesbrecht, T. (2013). Caffeine as an attention enhancer: reviewing existing assumptions. Psychopharmacology (Berl), 225(2), 251-274. doi:10.1007/s00213-012-2917-4
- Lieberman, H. R., Tharion, W. J., Shukitt-Hale, B., Speckman, K. L., & Tulley, R. (2002). Effects of caffeine, sleep loss, and stress on cognitive performance and mood during U.S. Navy SEAL training. Sea-Air-Land. Psychopharmacology (Berl), 164(3), 250-261. doi:10.1007/s00213-002-1217-9
- Smith, A. (2002). Effects of caffeine on human behavior. Food Chem Toxicol, 40(9), 1243-1255.
- Burke, T. M., Markwald, R. R., McHill, A. W., Chinoy, E. D., Snider, J. A., Bessman, S. C., . . . Wright, K. P., Jr. (2015). Effects of caffeine on the human circadian clock in vivo and in vitro. Sci Transl Med, 7(305), 305ra146. doi:10.1126/scitranslmed.aac5125