The Trump administration plans to shut down the federal health insurance exchange for 12 hours during all but one Sunday in the upcoming Obamacare open enrollment season.

The shutdown will occur from midnight until noon every Sunday except Dec. 10.

The Department of Health and Human Services will also shut down the federal exchange -- healthcare.gov -- overnight on the first day of open enrollment, Nov. 1. More than three dozen states use that exchange for their marketplaces.

HHS officials disclosed this information Friday during a webinar with community groups that help people enroll in Obamacare.

Outages occurred under the Obama administration, as well, but they weren't as frequent. Typically, the online enrollment system was only shut down for a few hours at a time and it was much less frequent than once a week, former officials said.

"There is just a really big question as to why this is happening," said Lori Lodes, former CMS communications director.

The current administration said that the maintenance schedule was provided in advance to accommodate requests from the groups that assist applicants.

"Maintenance outages are regularly scheduled on healthcare.gov every year during open enrollment. This year is no different," a spokesperson said. "System downtime is planned for the lowest-traffic time periods on healthcare.gov including Sunday evenings and overnight."

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The Trump administration has come under attack from critics who say that it is intentionally undermining the Affordable Care Act through regulatory actions. It cut the enrollment period, slashed money for advertising by 90% and sliced the budget for navigators to help people shop for plans.

The fact that HHS is now closing the site for a portion of each weekend upset many consumer advocates. Many working Americans — the prime target group for ACA insurance — might be shopping at just that time.

"Argh," was the reaction of Shelli Quenga, program director at the Palmetto Project in South Carolina, a nonprofit group that received about $1 million to help with outreach and enrollment in the past 12 months. This month, HHS cut her budget in half for this year's open enrollment.

Open enrollment season this year runs from Nov. 1 to Dec. 15, less than half the time people have had to sign up during the first four years of the exchanges created under the Affordable Care Act.

More than 12 million people enrolled on the state and federal marketplaces for 2017 coverage, including more than 9 million on the federal exchange. Some customers drop out over the course of the year as they get jobs or stop paying premiums.

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Advocates were already nervous that fewer people would sign up during the shortened period this time around.

"I could see this really impacting the ability of people to complete an application sign-up in a single sitting, which is so important," said Jason Stevenson, spokesman for the Utah Health Policy Project, an Obamacare navigator group. He noted that 10 p.m. Mountain Time is often a relatively popular time for people to enroll online.

"Health insurance is complicated, and in the past couple of years we had an administration that made it easier to sign up," he said. "But that has really changed in the past six months, with more hurdles not only for consumers but for those whose job it is to help them."

--CNNMoney's Tami Luhby contributed to this report.

Kaiser Health News is a national health policy news service that is part of the nonpartisan Henry J. Kaiser Family Foundation.