Bryson informed President Barack Obama that he was taking a medical leave "so that I can focus all of my attention on resolving the health issues that arose over the weekend," according to a statement released by the department. Deputy Secretary Rebecca Blank will serve as acting commerce secretary in Bryson's absence.
White House press secretary Jay Carney said in a statement that Obama's thoughts were with Bryson and his family.
The 68-year-old former utility executive struck a car stopped for a train -- twice -- on Saturday afternoon and then rammed into another vehicle with his car a few minutes later. He was found unconscious in his vehicle, and government officials said Monday he had had a seizure, which could play a role in whether he's charged with felony hit-and-run.
It wasn't clear whether the medical episode preceded or followed the collisions, but Bryson hasn't suffered a seizure before, said a department official who spoke on condition of anonymity to discuss the secretary's medical history. Bryson has a "limited recall of the events," the official said.
The crashes drew attention because of health concerns involving a member of Obama's Cabinet, as well as the challenge investigators face when trying to determine whether someone should be held criminally responsible because of adverse health.
Bryson was driving alone in a Lexus in San Gabriel, a community of about 40,000 northeast of Los Angeles, when he struck the rear of a vehicle that had stopped for a passing train, authorities said.
He spoke briefly with the three occupants and then hit their car again as he departed, investigators said. They followed him while calling police.
He was cited for felony hit-and-run, although he has not been charged.
Bryson then struck a second car in the nearby city of Rosemead, where he was found unconscious in his car, authorities said.
Bryson has returned to Washington after a brief hospital stay, department spokeswoman Jennifer Friedman said.
Officials said Bryson was not on state business, was driving a personal car and did not have a security detail at the time.
He took a Breathalyzer test that didn't detect any alcohol, but investigators were awaiting the results from a blood test, said Los Angeles County sheriff's Capt. Mike Parker.
Commerce officials said he was given medication to treat the seizure. Paramedics treated two people in the first collision for pain, but a couple involved in the second crash declined medical aid.
The case was being reviewed by sheriff's investigators and will likely be submitted to prosecutors in the coming days.
"In most cases, it is presented to the DA's office to make a decision," sheriff's Lt. Margarito Robles said.
Defense attorney Steve Meister, who has represented people who have been involved in crashes while having seizures, said "it's difficult to assign criminal liability when someone was medically unconscious. They have to be aware what was happening."
The episode is consistent with someone who has suffered a series of epileptic seizures, said Dr. Jerome Engel Jr., a neurologist at the University of California, Los Angeles, who is not involved in Bryson's care.
After a seizure, a person is often confused, and that state of confusion can last for a while.
"You may even seem to be alert and awake, but you're not really behaving normally," Engel said.
Under California law, a doctor has to report a patient who complains of lapses of consciousness or whose epileptic seizures pose an impairment to driving. In those cases, a person can't drive unless he's been seizure-free for three months.
Bryson had been in California to deliver the commencement address Thursday at Pasadena Polytechnic School, where his four children attended. The K-12 school said he urged students to pursue their passions, to serve their country, and to value their education and friendships.
Bryson was sworn in to lead the Commerce Department in October after easily overcoming conservatives' objections that his pro-environmental views made him unsuited for the job.
As secretary, Bryson is a member of the president's economic team and has advised on energy issues. He is the former head of Edison International, the holding company that owns Southern California Edison, and has served on boards of major corporations, including the Boeing Co. and the Walt Disney Co.
He helped oversee Edison's transformation into a leading wind and solar company and launched a plan to turn 65 million square feet of unused commercial rooftops into solar power stations with enough electricity for more than 160,000 homes.