Uncontrollable, even violent, shaking happens to about half of the 25 million people who undergo surgery each year as they are waking up, but the apparent shivering is not because the patient is cold, as has been thought for decades, University of California at San Francisco researchers report.

Instead, it now appears that the general anesthetics used to make the patient unconscious wear off at different rates in the spine and the brain. The spinal cord "wakes up" faster than the brain, creating "a condition where the spine is chemically disconnected from the brain, and leading to abnormal reflexes," according to a statement released by the researchers.

The researchers discovered that the tremors resembled a condition called clonus, an abnormal spinal reflex action that causes shaking in people with severed spinal cords. The cord initiates a reflex muscle action that becomes a continual shaking because the brain cannot shut it off.

Doctors have had a hard time figuring out what's going on because surgery patients are cold, too. During surgery, the patient's body temperature frequently drops, sometimes as much as 6 degrees Fahrenheit, because he is only partly covered, he receives cold intravenous fluids, and cold air is pumped into his lungs.

"These people are cold. They have a tremor that looks like a shiver," said Dr. Daniel I. Sessler, an anesthesiologist who led the study. "That is why everyone thought it was shivering."

Anesthetics, however, upset the body's ability to regulate its internal temperatures. The drugs, in essence, prevent the body from shivering from cold.

To make matters even more confusing, the treatment for clonus shivering is heating the skin with a sun lamp. Sessler's group discovered that any skin stimulus -- quickly bending an arm or a leg, coldness -- can stimulate the shaking.

"Warming the skin stops the tremor immediately, even while the core temperatue is very low," Slesser said.

Post-surgical shaking -- which can last 15 to 30 minutes -- can have severe consequences: It increases the metabolic rate, as much as twofold, straining the heart; it also pulls apart incisions, damages delicate surgical repairs and even breaks teeth.