Researchers want to know why breast cancer is more common in industrialized societies, particularly among shift workers (Breast cancer mystery frustrates scientists). One theory points the finger at increased exposure to artificial light, which disrupts the body’s natural sleep and wakefulness rhythm. The change in rhythm affects hormone levels, and this can trigger cancer, scientists believe.

This theory parallels the Chinese medicine view. It states that the energies present during daytime (Fire and Earth) favour activity, and the energy present during nighttime (Water) favours rest. When activity and rest are balanced, the corresponding energies in the body are more likely to be balanced as well.

In industrialized societies, the emphasis on productivity diminishes the amount of sleep we get, which unbalances the activity/sleep cycle and thus the energies in the body. This makes us more susceptible to certain diseases, including cancer (caused by excessive Earth).

When we reverse the activity/sleep cycle, as happens with shift workers, our bodies have to fight the natural energies of the time of day, so both activity and sleep suffer. This further increases our susceptibility to those diseases.