I used to have (strong) coffee at 7.30 am, and another at 11 am. But in the summer (out of routine) I was astonished that I was getting blinding headaches mid-morning if I hadn't had my coffee, which then lasted all day. So I switched to decaf for the first coffee.
But it still happens if I miss my mid-morning coffee. It is a real problem as I work in school, and sometimes I'm just too busy to grab a drink at breaktime; and yr9 set7 with a splitting headache is no fun at all.
Sorry I'm rambling - I don't understand why I'm so reluctant to just ditch the caffeine completely.