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Modulo-T and 24 h double-plotted actograms of representative hamsters in T30 (A, B) or T18 (D, E) under conditions of decreasing proportion of light to dark. Periodogram plots (C, F) for each photoperiod manipulation are shown for the corresponding portions of the actograms (red lines mark photoperiod transitions. These actograms and periodograms demonstrate that entrainment to T30 and T18 is only seriously compromised when the fraction of light to dark is substantially decreased.
