Abstract: A medical apparatus to be worn in the mouth comprises a battery-fed measuring unit (15) including a clock generator (18) and a counter (20) for counting the pulses of the clock generator. Accuracy of the time measurement decisively depends on the precision of the oscillating frequency of the clock generator. In the invention, there is used a clock generator (18) wherein frequency precision is low. When the counted value of the counter (20) is read into an external evaluating unit, the measuring unit supplies a reference time required by the clock generator (18) for generating a number of pulses. The evaluating unit calculates the time corresponding to the counted value under consideration of the length of the reference time. Thus, the evaluating unit measures the cycle length of the pulses of the clock generator, and this cycle length is multiplied by the number of the counted clock pulses. In this manner, time measurement is performed in a precise manner without requiring a highly precise clock generator.