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Referee Comments: Referee 1

Posted by PLOS_ONE_Group on 21 Aug 2007 at 17:39 GMT

Reviewer 1's Review

“The authors of this paper by Scheer et. al., investigate the plasticity of the circadian oscillator to previous light treatment. This is also known as "after-effect". It is well established that in non-human organisms, previous light treatment could modify the period length of the circadian oscillator for several weeks after the end of the light regimen and the start of constant dark conditions. The authors also tested two light regimens with a relatively high intensity (~450 lux; ~1.2 W/m2) to determine if they could entrain the circadian oscillator to 23.5 and 24.65 hours days by delivering light at times that do not disrupt the sleep part of the subjects.

The light treatment for entraining humans to two different days worked quite well. The authors show that administering light at the wake part of the cycle could produce both advances and delays that are needed to adjust to 23.5- and 24.65-hour days. This could be easily applied to help the normal adjustment to the non-24 hours days. The data from this part of the work is quite convincing.

The other hypothesis that the authors tested concerned the change in the period length of the circadian oscillator. They showed a significant yet small (~6 minutes only) change in period length between the 23.5- and 24.65- hour days. Despite the small change in the period the outcome that light could affect the period length of the oscillator is convincing.

I believe that this paper is an important advance in understanding how to adjust to non-24 hour days without affecting sleep and to determine that the circadian pacemaker in humans in also affected by the previous length of the applied day.”

N.B. These are the general comments made by the reviewer when reviewing this paper in light of which the manuscript was revised. Specific points addressed during revision of the paper are not shown.