The dawn phenomenon means an increase of P-glucose concentration between midnight and 9 a.m. by at least 1 mmol/l and its development may influence the P-glucose daily profile. The aim of this prospective study was to assess the development of ISF-glucose concentrations in the morning hours to recognize the dawn phenomenon, to define the frequencies of individual ISF-glucose concentrations over 24 hours and to evaluate the quality of the CGMS sensor. The sensor was inserted for a period of 1 to 8 days into the subcutaneous tissue of 12 healthy volunteers (10 women and 2 men aged 27 ± 10 years (x ± SD), BMI 22 ± 1.5 kg/m2, BP 117 ± 10 over 75 ± 10 mmHg; heart rate 68 ± 5,4/min; fP-glucose 4.5± 0.41 mmol/l; HbA1C 4.9 ± 0.21 %). P-glucose was investigated by means of a glucose meter Optium and its value was stored together with the codes of other important events in the memory of the monitor. Sensors were kept inserted in the course of 1064 h and functioning in 1013 h; 12 159measurements were made incl. 513 paired values. Mean absolute error (sensor vs. glucose meter) was 14.0 %. The dawn phenomenon was recorded in 75 % of all volunteers, mostly (in 66.7 %) repeatedly. The frequencies of ISF-glucose concentrations are characterized by a normal distribution. More than 96 % of all values of ISF are found in a range from 3.0 to 8.0 mmol/l. The CGMS proved to be a safe method for the assessment of P-glucose and the recognition of the dawn phenomenon. Further studies are necessary to assess the importance of the dawn phenomenon in diabetic patients and non-diabetic individuals.

        Key words: continuous glucose monitoring, selfmonitoring, interstitial fluid, dawn phenomenon, healthy volunteers.