Circadian rhythm adjustment: difficulties in assessment caused by masking.

Measured circadian rhythms consist of endogenous (clock-driven) and exogenous (lifestyle-driven) components. The exogenous components cause difficulties of interpretation of phase and amplitude information. Such effects can be removed by "constant routines", but these protocols are restricted to the laboratory. By contrast, the process of "purification" of circadian rhythms uses the knowledge of an individual's normal lifestyle to separate a measured rhythm into its two components. This method has been shown to be practicable in a variety of circumstances and to provide a feasible alternative to constant routines.