My skin is very greasy, I drink about 4 big cups a day sipping constantly, to maintain the same level of caffeine to improve coding skills, but when I have long vacations (two weeks) I don't see any side effects like headaches and my skin is probably even greasier. Also I don't have any trouble sleeping with or without coffee, but I drink only until end of work (16:00 for me). So, everyone is different. If I don't drink coffee, I feel it very well when trying to code, I can even detect by effects if some cofee is low on caffeine.