Circadian rhythm changes in core temperature over the menstrual cycle: method for noninvasive monitoring

The purpose of this study was to determine whether core temperature (T-c) t elemetry could be used in ambulatory women to track changes in the circadia n T-c rhythm during different phases of the menstrual cycle and, more speci fically, to detect impending ovulation. T-c was measured in four women who ingested a series of disposable temperature sensors. Data were collected ea ch minute for 2-7 days and analyzed in 36-h segments by automated cosinor a nalysis to determine the mesor (mean temperature), amplitude, period, acrop hase (time of peak temperature), and predicted circadian minimum core tempe rature (Tc-min) for each cycle. The T-c mesor was higher (P less than or eq ual to 0.001) in the luteal (L) phase (37.39 +/- 0.13 degrees C) and lower in the preovulatory (P) phase (36.91 +/- 0.11 degrees C) compared with the follicular (F) phase (37.08 +/- 0.13 degrees C). The predicted Tc-min was a lso greater in L (37.06 +/- 0.14 degrees C) than in menses (M; 36.69 +/- 0. 13 degrees C), F (36.6 +/- 0.16 degrees C), and P (36.38 +/- 0.08 degrees C ) (P less than or equal to 0.0001). During P, the predicted Tc-min was sign ificantly decreased compared with M and F (P less than or equal to 0.0001). The amplitude of the T-c rhythm was significantly reduced in L compared wi th all other phases (P less than or equal to 0.005). Neither the period nor acrophase was affected by menstrual cycle phase in ambulatory subjects. Th e use of an ingestible temperature sensor in conjunction with fast and accu rate cosinor analysis provides a noninvasive method to mark menstrual phase s, including the critical preovulatory period.