What is it?
Caffeine is a stimulatory anti-sleep compound extracted from coffee beans. It is often the main ingredient for pre-workout supplements or energy drinks. Habitual caffeine use leads to tolerance, which dulls several of caffeine’s effects.

What does it do?
Caffeine inhibits adenosine receptors. Adenosine causes a feeling of relaxation and sleepiness when it acts on brain receptors. Caffeine prevents this action and causes alertness and wakefulness. This inhibition of adenosine can influence the dopamine, serotonin, acetylcholine, and adrenaline systems. Caffeine is a powerful stimulant, and it can be used to improve physical strength and endurance. It is classified as a nootropic because it sensitizes neurons and provides mental stimulation. 

Additional Reading and References:

Reviews

Performance and sports

  1. International society of sports nutrition position stand: caffeine and performance. - PubMed - NCBI
  2. The effects of a pre-workout supplement containing caffeine, creatine, and amino acids during three weeks of high-intensity exercise on aerobic and... - PubMed - NCBI
  3. Caffeine and anaerobic performance: ergogenic value and mechanisms of action. - PubMed - NCBI
  4. Caffeine use in sports. A pharmacological review. - PubMed - NCBI
  5. Acute effects of caffeine-containing energy drinks on physical performance: a systematic review and meta-analysis. - PubMed - NCBI
  6. Performance outcomes and unwanted side effects associated with energy drinks. - PubMed - NCBI (caffeine)
  7. Exercise and sport performance with low doses of caffeine. - PubMed - NCBI
  8. Performance effects and metabolic consequences of caffeine and caffeinated energy drink consumption on glucose disposal. - PubMed - NCBI
  9. The acute effect of caffeine supplementation on strength, repetition sustainability and work volume of novice bodybuilders
  10. The effect of caffeine on skeletal muscle anabolic signaling and hypertrophy. - Applied Physiology, Nutrition, and Metabolism
  11. Coffee and Caffeine Ingestion Have Little Effect on Repeated Sprint Cycling in Relatively Untrained Males
  12. Effects of coffee and caffeine anhydrous on strength and sprint performance. - PubMed - NCBI
  13. Impact of Caffeine and Protein on Postexercise Muscle Glycogen Synthesis
  14. Effect of a moderate caffeine dose on endurance cycle performance and thermoregulation during prolonged exercise in the heat

Long-term effects, tolerance and withdrawal

  1. Chronic ingestion of a low dose of caffeine induces tolerance to the performance benefits of caffeine. - PubMed - NCBI
  2. Caffeine tolerance and choice in humans. - PubMed - NCBI
  3. Caffeine Tolerance is Incomplete: Persistent Blood Pressure Responses in the Ambulatory Setting
  4. A critical review of caffeine withdrawal: empirical validation of symptoms and signs, incidence, severity, and associated features | SpringerLink
  5. The Cumulative Neurobehavioral and Physiological Effects of Chronic Caffeine Intake: Individual Differences and Implications for the Use of Caffeinated Energy Products
  6. Caffeine Use Disorder: A Comprehensive Review and Research Agenda

Cognitive function

  1. A Comparison of Blue Light and Caffeine Effects on Cognitive Function and Alertness in Humans
  2. Effects of caffeine and glucose, alone and combined, on cognitive performance. - PubMed - NCBI
  3. Effects of two doses of glucose and a caffeine-glucose combination on cognitive performance and mood during multi-tasking. - PubMed - NCBI

Sleep

  1. Effects of caffeine on the human circadian clock in vivo and in vitro

Energy expenditure

  1. Effect of acute caffeine ingestion on EPOC after intense resistance training. - PubMed - NCBI

Other