The free-running property of circadian clocks is needed to tolerate changing photoperiods

The hourglass timing system in cyanobacteria shows dysregulated transcription and fails to provide resistance to midday UV exposure.

• Cyanobacteria possess two timing systems: a circadian clock and an hourglass-like system.
• In balanced light-dark cycles, both timing systems perform similarly.
• The hourglass system struggles with dysregulated transcription under long photoperiods.
• A mathematical model indicates that hourglass systems cannot adapt to changes in day length, unlike free-running clocks.
• The findings suggest that a self-sustaining circadian rhythm is necessary for survival in environments with variable day length.

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