The parasympathetic nervous system is responsible for calming us down, and a substance called adenosine builds up in the brain and body as we stay awake. When we sleep, adenosine is pushed back down. Caffeine blocks the effects of adenosine, making us more alert. However, when the caffeine wears off, adenosine binds to receptors more strongly, causing an afternoon crash. To prevent this, delay the intake of caffeine by 60 to 90 minutes after waking to allow adenosine to be cleared out. Exercise also helps clear out adenosine. By hydrating and training in the morning, you can reduce the risk of an afternoon crash. Gradually increase the time between waking up and consuming caffeine to see significant benefits.