Although it has been recently noted that blood pressure variability is affected by the indoor thermal environment, these effects haven’t yet been studied in sufficient detail. Therefore, a field survey on ambulatory blood pressure and indoor temperature was conducted for 7 days in the winter month 2012. On the date that indoor temperature in the bedroom was lowest, morning blood pressure was higher and morning blood pressure surge was larger, compared to the date when temperature was highest. Furthermore, the blood pressure gap between the lowest day and the highest day was larger after getting out of bed than before.