I can see this in myself. I usually have about 48oz of coffee each day. Obviously less on some days and maybe more on others. On days that I don't drink coffee, I can't tell a difference in my alertness or energy. Honestly, I just really really love the taste, and knocking back a cup is a actually a great stress reliever for me.

Edit: Now that I think about it, my dad is the same way. I've watched him multiple times drink a cup or two before bed and have no problems sleeping. On the other hand, my mom can drink a 12oz cup and get a nice boost of energy.