Most people think of a "normal" body temperature as an oral temperature of 98.6° (37°). This is an average of normal body temperatures. Your temperature may actually be 1°F (0.6°C) or more above or below 98.6° (37°). Also, your normal body temperature changes by as much as 1°F (0.6°C) throughout the day, depending on how active you are and the time of day. Body temperature is very sensitive to hormone levels and may be higher or lower when a woman is ovulating or having her menstrual period.
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http://health.yahoo.com/infectiousdisease-symptoms/body-temperature/healthwise--hw198785.html)