Why does caffeine wake you up?
Caffeine has similar properties to adenosine, a nucleoside that causes drowsiness by slowing down nerve cell activity. Caffeine can therefore bind to adenosine receptors. When this occurs, the receptors are not available for adenosine molecules to bind to which causes increased neuron firing in the brain.
The pituitary gland sees all of the activity and thinks some sort of emergency must be occurring, so it releases hormones that tell the adrenal glands to produce adrenaline (epinephrine). Adrenaline is the "fight or flight" hormone, and it has a number of effects on your body including:
- Pupil dilation
- Increased breathing rate
- Increased heart rate
- Constriction of blood vessels to slow blood flow from cuts and to increase blood flow to muscles
- Increased blood pressure
- Decreased blood flow to the stomach, this is no time to spend energy digesting!
- Release of sugar from the liver into the bloodstream for extra energy
- Tightening of muscles as they get ready for action
This explains why after consuming a big cup of coffee, your hands get cold, your muscles tense up, you feel excited, and you can feel your heart beat increasing.