How critical is it for chickens to roost at night?

We are chicken sitting and the chickens won't go to the upstairs of the coop tonight. The upstairs part is closed off at night. The downstairs is covered with chicken wire. Is it critical that we get them up for their safety or can they stay in the lower part of the coop for one night? They can always go up if necessary, but it won't be closed off. Are they are greater risk to predators?