When you sleep with your mouth open or breathe through your mouth at night, you may notice that your basal body temperatures are lower than usual. This is not unusual but there is really no way to correct the chart to account for this. If this happens only occasionally (due to nasal congestion with a cold for example) this should not have much impact on your chart. Simply make a note of the circumstances so that you will remember why that temperature seems out of place. One or two unusual temperature points will not usually affect your chart interpretation. You will still likely be able to see your ovulation pattern.