Effects of Photoperiod on Rat Motor Activity Rhythm at the Lower Limit of Entrainment

The experiment found that rat motor activity patterns exhibit two circadian components depending on the photoperiod.

• One circadian component aligns with the external light-dark cycle, while another component operates on a free-running basis.
• The expression of these components varies with photoperiod length, showing a transition between them.
• Under longer photoperiods, the component that matches the external cycle is more pronounced.
• In contrast, the free-running rhythm is more prominent during shorter photoperiods.
• The free-running component's period is longer in a 22-hour cycle than in a 23-hour cycle.
• Longer photoperiods lead to an extended free-running component period for both cycle lengths.

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