Scientists don't know exactly what causes sundown syndrome. But studies have shown an association between sundown syndrome and changes in the internal biological clock among people with dementia. The internal clock -- governed by the circadian rhythms -- controls sleeping and waking, is connected to how active we are at different times of the day, and influences changes in the body that regulate behavior. Studies suggest that the biological clock shifts in people with dementia, and that shift may make some people with dementia more prone to sundown syndrome.

If someone is susceptible to sundown syndrome, researchers theorize that hunger, a drop in blood pressure after a meal (which temporarily takes oxygen away from the brain), or changes in glucose levels in the blood from eating in people with diabetes may bring on agitation and confusion. Other physiological influences include whether someone is able to hear or see well.

If someone is confused and has vision problems, it may affect how he sees things around him as day shifts into twilight. "We had a classic sundowning situation with a patient with macular degeneration [an eye disease that causes loss of central vision]. He was calling the police repeatedly and said that there were robbers in his house," says John E. Morley, a professor of geriatrics at the St. Louis University School of Medicine. A visit to the man's home revealed what was triggering the calls. "He had slats in the blinds on his window, and at sunset, sunlight came through and created stick figures that he thought were robbers coming into his house," he adds.