We encourage you to measure only once each day as your first reading after waking up is always the closest to your actual basal temperature.
Basal thermometers can measure small changes in your temperature. If you measure several times in a row, your body's temperature will naturally rise (and sometimes fall), so the first temperature reading will be different than the next reading. The basal body temperature changes with how long you have been awake, moving, if you stand up, etc. This is why you need to measure as soon as you wake up, before you snooze, lie in bed for a while, or get out of bed.