Caffeine is a central nervous system stimulant of the methylxanthine class. It is the world’s most widely consumed psychoactive drug.
Caffeine reversibly blocks the action of adenosine on its receptors and consequently prevents the onset of drowsiness induced by adenosine.
Natural sources include
A study done within the military on sleep-deprived soldiers demonstrated that caffeine significantly improved cognitive performance mood and alertness for 3 additional hours a day: https://www.ncbi.nlm.nih.gov/books/NBK209050/
For the general population of healthy adults, Health Canada advises a daily intake of no more than 400 mg. This limit was found to be safe by a 2017 systematic review on caffeine toxicology.
Although there are no absolute contraindications to caffeine, there are some medical conditions in which caution is necessary.
American College of Obstetricians and Gynecologists (ACOG) considers 200 mg daily safe during pregnancy.
Caffeine may have negative interactions with several conditions including heart disease, hypertension and liver disease.
Caffeine has several physical and psychological adverse reactions in some people including gastrointestinal reactions and dehydration as well as increasing anxiety.
1. Is Caffeine a cognitive enhancer? https://pubmed.ncbi.nlm.nih.gov/20182035/