A basal thermometer is a thermometer used to take the basal (base) body temperature, the temperature upon waking. Compared to daytime temperatures, basal body temperatures are much less affected by environmental factors such as exercise and food intake. This allows small changes in body temperature to be detected, such as those caused by ovulation or changes in thyroid function.
lass fever thermometers typically have markings every 0.1°C (0.2°F). Basal thermometers require accuracy of at least 0.05°C (0.1°F), so special glass basal thermometers are sold separately from glass fever thermometers. All digital thermometers generally read to at least 0.05°C (0.1°F) and so are suitable for monitoring basal body temperatures. Some digital thermometers are marketed as "basal thermometers" and have extra features such as a larger display, expanded memory functions, or beeping to confirm the thermometer is placed properly.