how long should you leave a thermometer under your tongue
1 You should keep your mouth closed during this time. 2  Place the thermometer tip under the tongue. 3  Hold the thermometer in the same spot for about 40 seconds. 4  Readings will continue to increase and the F (or C) symbol will flash during measurement.ormal body temperature is about 98.6 degrees Fahrenheit (Â°F) or 37 degrees Celsius (Â°C). Your temperature often varies from 1 to 2 Â°F or Â½ to 1 Â°C. Your temperature is usually lower in the morning and increases during the day.