The newest restrictions require people not on essential errands to stay home from 10 p.m. to 5 a.m. through Dec. 21, with a possible extension if rapidly worsening trends don't improve. People are allowed to shop for groceries, pick up food and even walk their dogs.
Authorities say the focus is in keeping people from social mixing and drinking -- the kinds of activities that are blamed for causing COVID-19 infections to soar after dipping only a few months ago.
Dr. Mark Cullen, an infectious disease expert who recently retired from Stanford University, said the underlying goal is based on a reasonable interpretation of data.
"Large numbers of people getting together oblivious of controls - no masks, no social distancing, often indoors - a lot of those things are in fact occurring at night," Cullen said. However, he also questioned whether a limited curfew will be effective.
The curfew applies to 41 of the state's 58 counties that are in the "purple" tier, including all of Southern California, the most restrictive of four state tiers allowing various stages of economic reopening. Those counties encompass 94% of the nearly 40 million people living in the most populous U.S. state.
California as a whole has seen more than 1 million infections, with a record 13,000 new cases recorded Thursday.
Los Angeles County, the state's largest, enacted its own restrictions Friday that mostly affect businesses like restaurants, breweries, wineries and other establishments deemed non-essential.
However, even more restrictions are possible if high case numbers persist.
Officials had said that if the county averages more than 4,000 newly reported cases a day for a five-day period or 1,750 hospitalizations, it would end dining entirely; restaurants would only be able to offer food for takeout and delivery. If cases or hospitalizations reach 4,500 or 2,000, respectively, the county will go on lockdown and impose a stay-at-home order for three weeks.
That could happen as early as Sunday but county public health Director Barbara Ferrer said businesses would be given several days' warning before the rules are enacted.
Health officials acknowledge that the curfew will help flatten the infection rates only if people heed it voluntarily. Violators could face fines or be charged with a misdemeanor, and businesses could have their business licenses revoked. But counties are mainly responsible for enforcement.
Officials in Los Angeles, Orange and San Bernardino counties were among those saying they would not enforce the curfew, with some strongly opposed.
The Associated Press contributed to this report.