Two Steady‐Entrainment Phases and Graded Masking Effects by Light Generate Different Circadian Chronotypes inOctodon degus

Two main chronotypes—diurnal and nocturnal—are observed in Octodon degus, with a potential third intermediate chronotype emerging from analysis.

• Diurnal degus show activity primarily during the light phase, while nocturnal degus are active during the dark phase.
• An intermediate chronotype may exist, indicating a spectrum of activity patterns rather than two distinct types.
• The entrainment of the diurnal pacemaker corresponds with the light phase, whereas the nocturnal pacemaker aligns with the dark phase.
• Light has a significant negative masking effect on nocturnal degus but less so on diurnal ones.
• Access to a wheel appears to influence the transition from diurnal to nocturnal behavior, suggesting a role for environmental factors in chronotype expression.
• Body temperature rhythms indicate that the circadian pacemaker contributes to differences in chronotype, with gradual changes observed rather than sudden shifts.

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