It's strange that the article hints at genetic differences in caffeine metabolism and sensitivity ("Some people will get edgy from a weak cup of tea. For others, a double espresso is required to get them into the shower in the morning.") but doesn't follow through on the implications. For example, my 23andMe results indicate that I'm a "fast caffeine metabolizer", which is associated with a neutral to slightly lower risk of heart attack due to caffeine consumption, whereas slow caffeine metabolizers have a higher risk. In other words, it's clear that the answer to the question "How Much Caffeine Is Too Much?" is "It depends on your genes."
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.
