There are rules for sleeping on WSG 65 but they are too simulationist.¹⁾

Instead, apply the following rules:^Ex

Any character wearing armor to sleep more than 1 night in a row takes a cumulative -1 to all to-hit rolls and saving throws for each night past that 1st night, up to a maximum of -4 (following the 5th consecutive night sleeping in armor). One night sleeping out of armor removes the penalty and resets the counter.

Any character sleeping in metal armor needs 12 hours of sleep instead of the regular 8. Elfin chain is exempt.