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Basal Body Temperature

To answer the key questions surrounding basal body temperature, we called upon Dr. Joseph Hasson, a MD Gynecology, Obstetrics & Infertility specialist, and our Chief Medical Officer.

What is Basal Body Temperature (BBT)?

BBT is defined as body temperature under basal conditions at rest - the lowest temperature our body reaches during a 24-hour period. This is the meaning of “basal” in BBT. This lowest temperature is usually reached during deep sleep, when our body’s metabolism is minimal so that minimal heat is produced.

What time of day should we measure BBT?

Our BBT is usually reached during deep sleep, when our body’s metabolism is minimal so that minimal heat is produced. Thus, measurement of temperature on awakening (before arising and before any activity) is by all means NOT the basal body temperature and is being done so only for practical purposes. Continuous monitoring of body temperature during sleep would identify the true BBT much better than a single measurement upon awakening. Moreover – continuous temperature measurements during sleep allows monitoring of many other parameters that may affect BBT such as length of sleep, body movements, short periods of awakening at night, and so on.

Where on the body should we measure BBT?

Measuring core body temperature accurately is best done by placing a thermometer as close as possible to the “core” of our body, which is at maximal proximity to the heart.

The best way to do this is by placing an internal catheter into the pulmonary artery which is one of the two major blood vessels that originate from the heart(1). This catheter is called “Swan-Ganz catheter” and its insertion is done only in hospital setting and even there only when intensive monitoring is required, as of course this is an invasive and delicate procedure.

In day to day life we need a more practical yet still accurate means of measuring our core body temperature. Studies have shown that continuous measurement of skin temperature at the axilla is as good as episodic axillary temperature measurement, even in a hyper-sensitive population such as cancer patients after chemotherapy treatments(2). Another study of intensive care patients compared a range of alternative devices with core body temperature measured at the pulmonary artery by the Swan-Ganz catheter, in order to identify the most valid and reliable technique for measuring temperature in routine conditions. The authors found that the gallium in glass thermometer placed in the axilla for 12 minutes was the most accurate from all thermometers used in this study. The digital axillary measurement was as accurate and it was shown that using the gallium in glass thermometer for 12 minutes was slightly more accurate as compared with using it for 5 minutes(3). Taken together, this data implies that the longer and more continuous one measures the axillary temperature the more accurate assessment of core temperature is obtained.

As skin temperature is affected by the environment temperature, care should be taken to measure the axillary temperature at the top of the axilla with the best thermal insulation possible. Of course, sleeping is a dynamic state which means good thermal insulation can’t be kept continuously. This is why Tempdrop gives special attention to the use of another built-in thermometer that measures accurately the close environment temperature without touching the skin. This data is also used by Tempdrop's algorithms to allow the best body temperature measurement possible.


  1. Fulbrook P (1993) Core temperature measurement in adults: a literature review. J Adv Nurs 18(9):1451–1460.
  2. Maarten van Vliet & J. Peter Donnelly & Carin M. J. Potting & Nicole M. A. Blijlevens. Continuous non-invasive monitoring of the skin temperature of HSCT recipients. Support Care Cancer (2010) 18:37–42
  3. Rubia-Rubia J. et.al. Measurement of body temperature in adult patients: Comparative study of accuracy, reliability and validity of different devices. Int J Nurs Stud 48 (2011) 872-80

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