(CNN) -- Insomniacs and chronic wee-hour shoppers: Take shopping for Obamacare insurance on the federally run HealthCare.gov website off your overnight shopping lists -- at least for a while.
The part of the problem-plagued website that allows people to apply for coverage will now be taken offline nightly between 1 and 5 a.m. ET, according to a banner appearing atop the site's home page.
"The Health Insurance Marketplace online application isn't available from approximately 1 a.m. to 5 a.m. EST daily while we make improvements," the banner reads. "Additional down times may be possible as we work to make things better. The rest of the site and the Marketplace call center remain available during these hours."
The nightly blackout comes amid furious efforts to fix the website and a new warning by the White House that initial enrollments are likely to fall short of expectations.
"I can promise you that the first enrollment numbers which will be released later this month are not going to be what we want them to be," senior administration adviser Dan Pfeiffer told the ABC show "This Week" on Sunday. "There's no question about that."
The website's myriad problems tarnished the October 1 launch of the website for President Barack Obama's signature legislative achievement, reforms that were meant to bring affordable health insurance to millions of people who either couldn't get coverage before or had to buy pricey policies on their own.
"During the campaign I made you a promise. I promised that by the end of my first term, I would have passed health care reform into law. I would have signed that bill and thanks to your help, we did that. We got it done," Obama said Monday in remarks to Organizing for Action, the political advocacy group closely aligned with his agenda.
"A lot of us didn't realize that passing the law was the easy part," he said, addressing the site's problems.
"I'm not happy about it, as you might imagine. I'm not happy about it because I know that people need health care and this is the right place to get it," Obama said.
"That's unacceptable, and I'm taking responsibility to make sure that it gets fixed, and it will be fixed. We're working overtime to get it fixed," Obama said.
Those problems -- alongside news of policy cancellations and premium increases for some on the individual market -- have given Obama's Republican critics ammunition in their fight against the Affordable Care Act reforms and its requirement that most U.S. citizens buy health insurance coverage or face penalties.
"It's time to call a timeout," Sen. Kelly Ayotte, R-New Hampshire, told CNN's Candy Crowley on "State of the Union" on Sunday.
After initially saying the problems were caused by overwhelming interest in Obamacare that swamped the website's computer systems, administration officials acknowledged the issues ran deeper and ordered a "tech surge" to address them.
The administration brought in an entrepreneur and former administration official, Jeffrey Zients, to lead the repair effort and recruited other experts from government and the private sector to assist.
According to the Department of Health and Human Services, troubleshooters doubled the number of computers dishing up Web pages to visitors and made other behind-the-scenes changes to make the site more reliable.
On Friday, officials said the effort was paying off, with website pages responding more rapidly and consistently.
Administration officials have promised that the site will be functioning well for the majority of users by the end of the month.
Open enrollment for coverage beginning on January 1 ends on December 15, but the enrollment period continues through March for those who want coverage that starts later in the year.