This data-set is from an old study (Wilson R, Newman E and Newman H. 1956. Diurnal Variation in Rate of Alcohol Metabolism. J Appl Physiol 8 556-558.), back from the times when it was OK to recruit some college freshmen to drink alcoholic beverages in the name of science (good luck in getting any IRB in the USA to let you do that today!).
This is a record of a diurnal rhythm in alcohol clearance, and the figure is from a pamphlet: Palmer JD 1983. Human Biological Rhythms. Carolina Biological Supply Company, Burlington NC.:
It shows why we can drink more in the evening than at other times of day - there is so much more alcohol dehydrogenase activity in the evening. I am not encouraging drinking here, but if you are into it and can be responsible about it, you can save some serious money by downing a single shot at dawn, according to this graph, or enjoy it more at night.
So, what do you think - does it matter at what time of day/night cops stop you to give you a breathalyzer test? Or your medical tests of various kinds?
And what do you think about the ethics of the study?
a) it was unethical to do this even back in 1956
b) it was OK according to the ethics of the day, but ethics evolves over time so it is unethical today.
c) it is ethical today, but the "ethics creep" of the IRBs has gone way over the line of common sense.