Most drunk driving arrests take place at night, often well after midnight. One reason for this is that many police officers engage in "cherry picking" — that is, the illegal practice of staking out bars and restaurants from about 10:00pm to "closing time" at around 2:00am, pulling cars over on some pretext as patrons leave and drive away.
It is during this period of time that the individual’s circadian rhythm is taking effect. This is the 24-hour biological alarm clock in each of our bodies, often noticeable when we experience "jet lag".
Researchers have found that individuals will perform more poorly in tests during the low point of the circadian rhythm — that is, during the hours after midnight and into the early morning.
Unfortunately, it is just such tests — called "field sobriety tests" — that officers use to determine whether a driver is intoxicated or not.
British physicians and psychiatrists reported that "the same blood alcohol level is associated with a significantly greater impairment of different aspects of psychological funtioning when achieved in the morning." "Circadian Variation in Effects of Ethanol in Man", 18 (Supp. 1) Pharmacology, Biochemistry and Behavior 555.
The researchers concluded that "the differences we have found (in field sobriety test performances)…must be attributable to circadian change and susceptibility of the body to its effect."