A circannual rhythm in unstimulated salivary flow rate when the ambient temperature varies by only about 2 degrees C.

OBJECTIVE: To determine whether a circannual rhythm in the flow rate of unstimulated whole saliva could be detected in persons living in a region of Sri Lanka where the average monthly temperature is virtually constant throughout the year.
DESIGN: Forty-six medical students, 26 male and 20 female, in Peradeniya collected saliva on 12 occasions about once per month from September 19, 2003 to September 28, 2004. A least-squares sine wave of the form: F(t)=M+A x(2 pi t/tau+phi), in which F is the flow rate at time t (in days), M the mesor or rhythm-adjusted mean value, A the amplitude, tau the period (365 days) and phi the phase shift in radians, was fitted to the data.
RESULTS: A statistically significant (p<0.05) circannual rhythm in salivary flow rate was detected with the following mean values and 95% confidence intervals: M=0.528 mL/min (0.514-0.543), A=0.0472 mL/min (0.0269--0.0674), phi=1.141 radians (0.704--1.578). The acrophase (peak value of the rhythm) was on October 14 (September 19--November 8). During the study, the average monthly temperature varied between 24.0 and 26.3 degrees C, with the peak temperature in April and minimum in December.
CONCLUSION: Even though the monthly ambient temperature variation in Peradeniya was very small, the peak temperatures occurred at the times of the lowest salivary flow rates, suggesting that a small change in ambient temperature (about 2 degrees C) in a warm climate may be sufficient to influence the unstimulated salivary flow rate. In conclusion, for saliva studies, which extend over a prolonged period, it may be important to take into account the circannual rhythm in salivary flow rate.