People who drink a lot of coffee are less likely to develop type 2 diabetes than people who drink smaller amounts, or no coffee at all. Because the effects can actually be stronger with decaffeinated coffee, it’s probably not caffeine.

But research as also shows that there are reasons to not drink coffee as well. Caffeine can raise your blood pressure . Quitting coffee or caffeine can lower your blood pressure and keep your heart from working as hard.  Caffeine alters the mood.  Many report being grumpy until they’ve had their morning coffee, and others feel lethargic when the caffeine begins to wear off in the afternoon.

http://www.healthline.com/health/food-nutrition/is-coffee-good-for-you#3