Heart attacks often occur in the morning. Epileptic seizures peak in the late afternoon. Asthma attacks get worse and more deadly between 11 p.m. and 3 a.m.

Researchers are finding that circadian rhythms, which cycle every 24 hours or so, drive virtually every system in the human body, from circulation and cognition to metabolism, memory and mood. And they play a big role in determining when we are most vulnerable to disease.

Chronobiology—the...