The federal Healthcare.gov site that most states rely on for their Affordable Care Act (ACA) exchanges will be shut down for 12 hours for five of the six Sundays during the upcoming open enrollment period, as well as overnight on Nov. 1, the first day of open enrollment.

CMS revealed the schedule Sept. 22 during a webinar with navigator groups that help customers sign up for health insurance through the exchanges. The agency said the outages are needed for regularly scheduled maintenance on the site and will be conducted during low traffic times between midnight and noon eastern time on Sundays.

Four Democrats—Sens. Brian Schatz of Hawaii, Elizabeth Warren of Massachusetts, Cory Booker of New Jersey and Chris Murphy of Connecticut—have sent a letter to the HHS Office of the Inspector General (OIG) asking the agency for an analysis “of the rationale for and the impact of these extended shutdowns.”

“It is inexplicable that the system—which will now be in its fifth year of operation—would suddenly require orders of magnitude more downtime for maintenance,” they wrote.

In the 2016 open enrollment period, Healthcare.gov was online during 99.9 percent of the open enrollment period, according to a federal report to Congress. The additional maintenance this year will have the site operational for 93 percent of the 2017 enrollment period, which has already been cut in half to 45 days by new leadership at HHS and CMS, ending on Dec. 15. The previous year, enrollment ran until Jan. 31.

“Since taking office, President Trump and his administration have made multiple policy changes that threaten to undermine the ACA marketplace—and the recent announcements of extensive shutdowns of Healthcare.gov for 'maintenance' appear to be part of that pattern,” the senators wrote.