51 In crewmembers of a transmeridian flight, diurnal outdoor exercise speeds up the GSK1349572 concentration resynchronization of the urinary 17-hydroxycorti costeroid circadian rhythm, compared with those without exercise.52 Masking effects The advantage of a rhythm with the shape of a cosine function was discussed above. However, the patterns of many circadian rhythms deviate from that

of an optimal cosine function. In many cases, a secondary peak or shoulder is observed in the 24-h pattern. This shoulder may indicate the presence of additional period component (eg, with τ<20 h), and the rhythm may be Inhibitors,research,lifescience,medical defined as a compound rhythm. However, the change may be due to masking effect. Masking is the result of a direct influence of one variable on another, or a direct influence of an external stimulus on a variable, without reference to a rhythmic process.48 In natural settings and habitual life conditions, the body temperature rhythm curve is trapezoidal rather than close to a cosine curve. Mills et al53 and Czeisler and Wright46 proposed a constant routine protocol, where the masking Inhibitors,research,lifescience,medical influences of ambient light, temperature, noise, food consumption, and activity level are carefully controlled. Subjects Inhibitors,research,lifescience,medical stayed awake in recliners for 24 to 48 h in dim light. In this condition, the unmasked rhythms of, for example, body temperature, exhibited a curve close to a cosine function. This type of experiment suggests that, in the real world, masking effects

may alter the curve of many circadian rhythms. However, it should be noted that the constant routine protocol, which involves Inhibitors,research,lifescience,medical sleep deprivation, might alter the circadian period of a set of variables and its adequacy for this study will be discussed in another section of this paper. Quantification

of rhythm parameters with special reference to τ In circadian rhythm studies, the critical parameter to be quantified is τ. In most investigations, it is assumed that τ=24 h (as Inhibitors,research,lifescience,medical a mean) when subjects are synchronized with a diurnal activity and nocturnal rest with stable and regular times (eg, awakening [lights on] at 7 am and retiring [lights off] at 11 pm). Using this procedure, a set of rhythms can be documented in subjects with a sampling interval of, for example, 4 h over a 24- or 48-h period. Using this transverse sampling, other circadian parameters can be computed, such as Φ, A, and 24-h M, provided the parameters exhibit statistically significant and rhythms. However, with a transverse sampling of this kind, 24-h rhythm is computed, but not the circadian τ. This can only be obtained by longitudinal sampling over at least 7 days. With these requirements, inter- and intraindividual changes can be taken into account, which is mandatory to document human rhythms in certain circumstances. Prominent τ with the largest A, as well as other periods (with lower As) are quantified from time series by relevant methods including power spectra .