Reader Comments

Post a new comment on this article

Referee Comments: Referee 2

Posted by PLOS_ONE_Group on 21 Aug 2007 at 17:29 GMT

Reviewer 2's Review

“The authors applied a model-based lighting regimen to test the entrainment of the human circadian pacemaker to a non-24-h light-dark cycle. They report a successful entrainment to 24.65 h and 23.5 h cycles and demonstrated an after-effect during the subsequent 28-h forced desynchrony period.
The study was performed by the premier group of human circadian research who is unsurpassed with respect to methodology and analysis. The most important finding is the demonstration of an after-effect of a 2-week exposure to light-dark cycles which are either longer or shorter than 24 h. This is the first time that an after-effect on the human circadian system has been demonstrated. In their pioneering studies, Pittendrigh and Daan had demonstrated this effect in rodents already in 1976. In the present study, the difference in the period length induced by the schedules shorter and longer than 24 h (Fig. 3) is consistent, but small (0.1 h). It would be useful for the general reader, if the authors would discuss their finding in more detail, comparing its magnitude and time course to the animal data. In rodents the effect persisted for a considerable time but became gradually wekaer. The few days on which the period estimate in the human study is based does not allow any statement on the time course. Nevertheless, the paper would gain from a discussion of this aspect.

Concerns and questions:
1. The repeated reference (abstract, text) to the 24.65-h Martian solar day in view of a prospective Mars mission is questionable and needs revision. Astronauts reaching Mars will be exposed to full sunlight and there should be no problem to entrain the endogenous 24.18-h rhythm (please provide this exact figure on p.4, ref. 12) to the slightly longer Martian day. The inability to entrain to the two light-dark cycles used in the present study has been observed (ref. 13) for conditions of very dim light. This is not the case in space travel. In the study reported in ref. 9 the lighting conditions were highly variable. The lack of entrainment to the 23.5 h work-rest schedule (which was also variable) was probably due to various factors. Therefore the relevance of the present findings to space travel is limited.”

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.