Circadian rhythms govern approximately 24-hour cycles in mammals. Which description matches diurnal, nocturnal, and crepuscular activity?

• A. Diurnal: active at night; Nocturnal: active during the day; Crepuscular: at dawn or dusk.
• B. Circadian rhythms are random and not linked to day-night cycles.
• C. Circadian rhythms are ~24-hour cycles; diurnal animals are active by day, nocturnal at night, crepuscular at dawn/dusk.
• D. Diurnal: active at noon.

Answer: (C)

Diurnal, nocturnal, and crepuscular describe when animals tend to be active within the daily cycle governed by circadian rhythms. Circadian rhythms are endogenous ~24-hour clocks that align with the day-night cycle, so animals have regular activity windows. Diurnal species are active during daylight hours, nocturnal species are active at night, and crepuscular species are most active at dawn and dusk. The statement that circadian rhythms are ~24-hour cycles and that diurnal means daytime activity, nocturnal means nighttime activity, and crepuscular means dawn/dusk activity captures both the timing and the rhythm.

The other descriptions misstate these ideas in different ways: one reverses the times (active at night for diurnal, active during the day for nocturnal), another treats circadian rhythms as random, and another narrows diurnal to noon, which misses the broader daytime activity and the crepuscular period.