When bedtime comes, as all children and sick people know, the boogeymen come out of the closet. Darkness, silence and isolation can turn chairs into tigers, and make even trivial health problems seem ominous and hopeless. But illness that goes bump in the night may not be just a patient's inner child coming out. Doctors have sensed for centuries that many diseases actually do get worse at night, and science has begun to confirm this impression. The emotions nighttime elicits, although they certainly do not help matters, are not primarily to blame. Instead, it is the body's internal chemicals and...