[Andreas]

A Will roll might be appropriate if the player want the character to stay in uncomfortable conditions despite there being an affordable alternative. Otherwise it would probably not be a good idea to assign penalties for mental weariness unless the character has an applicable disadvantage or the conditions truly are very bad. Plenty of people have after all spent their entire lives under conditions which by modern standards are rather bad.

If the conditions are bad enough to warrant such penalties (how bad the conditions have to be should probably vary between characters, for example someone with Temperature Tolerance or high HT might suffer much less from cold weather), I would suggest something like this.

Each day they have to make Will roll. If they fail, they get a -2 penalty to IQ (including Will and Per) due to the distraction of their weariness. For at most one hour per day, a character can halve this penalty if they first pass a Will roll, this represents the ability to focus past some of the weariness during important situations.

The penalty persists until they have the opportunity to rest under significantly better conditions or until they pass their daily Will roll with a margin of success greater than the largest margin of failure during the same penalty period.