There are actually several hundred scientific books and articles that refer specifically to seven-day cycles in nature. Although a whole host of life forms have well-defined weekly cycles, most of the research has been medical. Scientists have documented bona fide weekly fluctuations in human blood pressure, heart rate, blood chemistry, body temperature, kidney function, tooth development, urine chemistry and volume, stress hormones, various neurotransmitters, and the immune system.
Weekly rhythms appear easiest to detect when the body is under stress, such as when it is defending itself against a virus, bacterium, or other harmful intruder. For example, cold symptoms (which are really signs of the body defending itself against the cold virus) last about a week. Chickenpox symptoms (a high fever and small red spots) usually appear almost exactly two weeks after exposure to the illness.
Doctors have long observed that response to malaria infection and pneumonia crisis peaked at seven days. Organ transplants face similar crises as the body's immune system attack the foreign organ.