I agree with the caffeine comment. Believe it or not there is a balance when it comes to coffee. Having a cup of tea/coffee every day is not the same as having a cup every 2 hours. As in everything, it is not strictly an on/off switch.
Other than that I totally agree that sleep exercise and food make a great difference. Eating fat with carbs for 3 meals a day will not help you get the mental focus either.