Day length changes the timing difference between activity start in living animals and molecular clock peak in tissue samples

Photoperiod significantly changes the timing of the mPer2::luc peak relative to light offset and activity onset in mice.

• Different light-dark cycles (L:D 16:8 and L:D 8:16) alter the phase relationships between biological rhythms and the light cycle.
• The mPer2::luc peak maintains a stable phase relationship to activity offset while altering the relationship to activity onset in both photoperiods.
• No significant differences were found in the period, phase, or peaks per cycle between anterior and posterior slices of the suprachiasmatic nucleus (SCN) under the same photoperiod.
• After short-photoperiod treatment, anterior SCN slices exhibited increased amplitude of Per2::luc waveforms, while posterior slices showed shorter peak width.
• In vitro SCN tissue did not show period differences attributed to photoperiod pretreatment, indicating that behavioral rhythm aftereffects may not persist at the cellular level.
• The change in phase relationship to activity onset suggests that Per2::luc rhythms may track activity offset instead.

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