The Human Circadian Clock's Seasonal Adjustment Is Disrupted by Daylight Saving Time

A survey of 55,000 people shows that sleep timing follows seasonal dawn progression under standard time but not under daylight saving time.

• Circadian clocks synchronize to environmental daylight, but this synchronization is disrupted by daylight saving time (DST).
• Sleep and activity timing for individuals easily adjusts to the end of DST in autumn, but does not adjust well to the start of DST in spring.
• Late chronotypes experience significant difficulty in adjusting their activity timing to the spring DST transition.
• The human circadian system's inability to adjust to DST suggests potential disruptions to seasonal biological rhythms.

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