Editing Phase response curve (section)

===Additive effects===
In a 2006 study Victoria L. Revell ''et al.'' showed that a combination of morning bright light and afternoon melatonin, both timed to phase advance according to the respective PRCs, produce a larger phase advance shift than bright light alone, for a total of up to 2{{frac|1|2}} hours over the course of 3 days. All times are approximate and vary from one individual to another. In particular, there is no convenient way to accurately determine the times of the peaks and zero-crossings of these curves in an individual. Administration of light or melatonin close to the time at which the effect is expected to change sense abruptly may, if the changeover time is not accurately known, produce an opposite effect to that desired.<ref>{{cite journal | vauthors = Revell VL, Burgess HJ, Gazda CJ, Smith MR, Fogg LF, Eastman CI | title = Advancing human circadian rhythms with afternoon melatonin and morning intermittent bright light | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 91 | issue = 1 | pages = 54–9 | date = January 2006 | pmid = 16263827 | pmc = 3841985 | doi = 10.1210/jc.2005-1009 }}</ref>