Caffeine

Caffeine reduces sleepiness and fatigue while increasing alertness, energy, motivation, and arousal. 1 2

• About 90%: An estimated 90% of adults worldwide consume a caffeinated beverage every day, making caffeine the world’s most popular drug. 3
• Adenosine blockade: Caffeine occupies adenosine receptors, blocking sleepiness; when caffeine wears off, accumulated adenosine can bind and bring on a pronounced “crash.” 4
• Reinforcement and preference: Caffeine stimulates dopamine and acetylcholine release in the forebrain, supporting focus and alertness. Its reinforcing effects can also make people favor the flavors and routines associated with caffeinated foods and drinks. 5 6
• Mood and neuroprotection: Caffeine is associated with neuroprotective effects, and large-scale analyses find regular intake inversely related to depression—though the transcript notes the relationship may be direct or indirect. 7
• Dopamine receptors, adrenaline, and vision: Regular caffeine intake can increase the number and efficacy of dopamine receptors. Caffeine also increases adrenaline release; associated pupil dilation can make the visual field feel narrower and more tunnel-like. 8 9
• Timing and moderation: Keep caffeine early enough that it does not impair sleep; the guidance cited is to avoid it within 8–12 hours of bedtime, since sleep architecture—including slow-wave and REM sleep—may be disrupted even if you can fall asleep. 10