Many people know that caffeine is a strong diuretic - it makes you urinate more than usual. Apparently this is due to increasing the blood flow through the kidneys. It can produce insomnia - delaying the onset of sleep and reducing total sleeping time. It has a small effect on respiration by increasing blood flow through the lungs and increasing the supply of air by relaxing bronchiolar and alveolar smooth muscle. That's why it's proving effective in treating the breathing problems of some prematurely born infants.
Some people experience tremors after drinking coffee and tea.
That's thought to be due to over-activation of the central nervous system. And I'd better have a big cup of extra strong black coffee to get me past the breathalyser. Unfortunately, this is one of the most enduring myths about caffeine. On the other hand caffeine is a good friend the morning after. Alcohol can give you a thumping headache by enlarging cranial blood vessels.