Opposition MPs chose a night of sleep over a night of debate on Monday.

They were planning to force the House of Commons environment committee to sit all night in order to bring an end to what they are calling a government filibuster on a private member's bill, in this case one that was introduced by NDP Leader Jack Layton.

But instead of bringing in the mattresses, all opposition members stood up and walked out around 7 p.m., which broke the committee's quorum and ended three and a half hours of non-stop talk by Conservative MPs.

The opposition hasn't abandoned its committee sleepover. They just needed more time to plan an all-night session, possibly for tonight, Wednesday. They expect support from environmental groups who are bringing in their sleeping bags. The NDP has even arranged a room where members can go to catch a nap ane and eat.

"We are preparing for a fight," says NDP MP Nathan Cullen.

The idea is to simply wear down the Conservative MPs. For eight committee meetings in a row, a total of nearly 17 hours, the government members have run out the clock, denouncing Bill C-377.

The bill commits Canada to aggressive cuts to greenhouse gas emissions after the Kyoto period ends in 2012.

The Conservative MPs deny that what they are doing is a filibuster. They say they simply have problems approving a bill that hasn't been properly costed or analyzed.

The opposition accuses the government side of being philosophically opposed to anything that imposes tough emission targets on Canada.

They also suspect the delaying tactics are part of a larger plan to gum up the legislative works so the Conservatives can claim Parliament is bogged down and make the case for an election to clear the air.

With both sides not budging, Wednesday could prove to be a very long day. And night.