Introducing tougher Covid restrictions in the west of Scotland now could help pave the way to easing the rules over Christmas, Nicola Sturgeon has said.
The first minister believed it was "likely" but "not inevitable" that some areas would be moved into level four.
She said infection rates were still "stubbornly high" in some areas.
And that could lead to "less flexibility" for some limited easing of restrictions over the Christmas period - something she was "very keen to do".
She said the rates of infection also meant that the Scottish government "do not have as much assurance as we would want" that hospitals and intensive care facilities would be able to cope over the winter.
Ms Sturgeon said that moving to level four restrictions "for a limited period in some areas" would address both of those concerns.
She explained: "Where we have stubbornly high prevalence, if we want to protect our NHS and if we want to get to a point at Christmas where we might be able to have a bit of easing of restrictions, albeit that will be very careful, then we need to get that prevalence down more right now."
Questioned at her daily briefing, she said: "I don't want to get ahead of ourselves here - because there is lots of consideration and discussions, not least across the four nations of the UK right now - before we get to a settled point before Christmas.
"But if you are asking me my priority for Christmas it is to allow families some ability to get together.
"That should be the priority, and if we do go to level four for any areas tomorrow then part of it, not the whole reason, is to try to get prevalence down to the point where we think we can have some limited easing around that."
Ms Sturgeon said the council areas in Greater Glasgow and Clyde, with the possible exception of Inverclyde, were causing the most concern, along with North and South Lanarkshire.
Level four may well be coming for the west of Scotland. All the government indicators suggest it should be, but the reluctance until now can perhaps be explained by more than just economic consequences.
The effects of loneliness and isolation on people's mental health are already at alarming levels, doctors tell me.
Closing cafes where people who live on their own at least have a chance to meet someone will exacerbate that.
It will likely put even more pressure onto GPs, with community support services unable to operate in the same way.
And then for shielders. The governments strategic framework says that level four would see the chief medical officer write an automatic two week fit note for those in the shielding category.
That could affect staffing in the NHS or in schools and other vital services.
Later, she added: "I think it is likely that we will see some areas go to level four this week.
"But is it inevitable? Until we have taken that final decision, no of course it's not."
The next decision about restriction levels will be made on Tuesday, with any changes taking effect from Friday.
In level four bars and restaurants, non-essential shops, gyms and indoor sports facilities would close - but schools would remain open.
Prof Linda Bauld, an expert in public health at the University of Edinburgh, said she thought it was "highly likely" that large parts of central Scotland would be placed in level four.
She said the rolling average of cases per 100,00 people was about 143 in Scotland.
"For Greater Glasgow and Clyde it is up to 247. That is very high - and Lanarkshire is just below that at 241.
"So action clearly does need to be taken," she said.
The latest figures show that a further 717 people have tested positive for Covid-19 in Scotland, and six more deaths have been recorded.
Ms Sturgeon added that an era was "firmly on the horizon" where better therapies, vaccines, testing and treatments would be available.
"The end is not quite with us, but we can see hope on the horizon now that we couldn't see just a few weeks ago," she added.