Low doses can result in psychological effects of "mild euphoria, alertness, and enhanced cognitive performance"; higher doses produce physiological side effects of nausea, anxiety, trembling, and jitteriness. There are individuals who are prone to caffeines anxiogenic effects whilst others are susceptible to its caffeine-induced sleep disturbances and insomnia. Studies with twins have shown that genetics influence individual differences in response to caffeine. Homozygous twins have been found to react in more consistent ways to the caffeine than heterozygous twins. Behavioral effects
Caffeines widespread appeal is due primarily to its mild psychostimulant properties, which increase alertness and cognitive arousal and diminish fatigue. Caffeine also produces a wide range of other symptoms, including upregulation of the cardiovascular system, increased global cognitive processing, and improved physical functioning. Cardiovascular effects can range from increased heart rate and reactivity to severe cardiac arrhythmia. The cognitive effects of caffeine include increased performance in memory, attention, and text reading. The physical effects of caffeine include lowered sensations of pain, less perceived effort, and increased muscle strength and endurance. However, at doses above ~400 mg, both cognitive and physical performance begins to worsen and symptoms of anxiety increase. One study that explored ad lib caffeine consumption in 159 college students found that high consumers reported lower academic performance. The same study also concluded that moderate and high consumers combined reported significantly higher trait anxiety and depression scores when compared with abstainers. These anxiety symptoms include persisting insomnia, nervousness, and mood fluctuations.When undergoing stress, the body activates a system-wide response mechanism known as the HPA axis.
Prevalence
Although exact rates of prevalence are not available, general population data shows a 0.002% prevalence over a year-long period and higher prevalence within clinical populations. Caffeine
Caffeine is a methylxanthine, and is hydrophobic. The structure of caffeine allows the molecule to pass freely through biological membranes including the blood-brain barrier. Absorption in the gastrointestinal tract reaches near completion at about 99% after only 45 minutes. Half-life of caffeine for most adults is between 2.5 and 4.5 hours when consumption is limited to less than 10 mg/kg. However, during neonatal development, half-life for the fetus is significantly longer and decreases exponentially after birth to reach a normal rate at about 6 months. Cytochrome P-450, a hemeprotein, acts in liver microsomes to metabolize caffeine into dimethylxanthines, monomethylxanthines, dimethyl uric acids, monomethyl uric acids, trimethylallantoin, dimethylallantoin, and derivatives of uracil. Most caffeine is metabolized by 3-methyl demethylation, forming the metabolite of paraxanthine. Many metabolites, in addition to caffeine, act within the body and are partly responsible for the physiological response to caffeine. Mechanism of caffeine action
Caffeine acts in multiple ways within the brain and the rest of the body. However, due to the concentration of caffeine required, antagonism of adenosine receptors is the primary mode of action. The following mechanisms are ways in which caffeine may act within the body, but depending on necessary caffeine concentration and other factors may not be responsible for the clinical effects of the substance.