First you should survey each of potential test subjects by asking them how often they drink coffee. Then divide those who drink one cup from those who drink twos cups a day.
Have your test subject volunteer to skip,a day without coffee for the entire day.
During that one day, have your test subjects write down how they feel hour by hour .
After at least one day takes notes about their experiences.
Compare the two groups. How their day suffer? What symptoms did they daily coffee drinkers experiences?