24-Hour Rhythms at Several Levels of Integration in Mice on Different Lighting Regimens. ∗

Summary and conclusions

The following variables were studied in the mouse under conditions standardized for evaluation of 24-hour periodicity: glucose and “corticosterone” in blood; mitoses in pinnal epidermis and hepatic parenchyma; liver glycogen and the hepatic metabolism of cytoplasmic phospholipid and ribonucleic acid and of nuclear deoxyribonucleic acid. For each variable, data obtained with light from 6 p.m. to 6 a.m. alternating with 12 hours of darkness, were compared with data obtained with light from 6 a.m. to 6 p.m. A shift in timing of 24-hour rhythms, following the inversion of lighting regimen came clearly and consistently to the fore. Ordinarily, environmental lighting is the dominant synchronizer of various 24-hour rhythmic functions, at different levels of organization of mice: by instituting an appropriate schedule of lighting and by allowing thereafter for the necessary shift time, one may set the peak or trough of rhythm to any chosen clock hour. Moreover, by controlling the timing of various periodic functions with respect to the environment, defined phase relations are brought about among the various physiologic rhythms themselves. The possibility to control timing of these rhythms by an easily manipulated environmental factor is of obvious practical interest, but it also constitutes a requisite for a basic analysis of functions, within the range in which they vary under ordinary circumstances.