Seasonal clock changes continue to serve an important role in aligning human activity with seasonal variations in ambient light, according to new findings.
A recent study analyzed the physiologic and social function of seasonal daylight saving time (DST) in midlatitude societies. Study authors, José María Martín-Olalla and Jorge Mira, of the Universidad de Sevilla, Spain, offered a counterpoint.
In the study, published in Royal Society Open Science, they reviewed the acute, short-term impacts and proposed chronic, long-term effects of daylight-saving time in the context of changing light conditions throughout the year.
The investigators highlighted that sunrise, rather than solar noon, is a primary cue for human circadian rhythms, especially during winter. At extratropical latitudes, the phase of human activity tended to shift later in winter in response to delayed sunrise times.
"We maintain that with increasing latitude there is a pressure to delay (the phase of) human activity in winter, associated with the delay of sunrise times and to our photoreceptive mechanisms that set alertness. We sustain that this stressor ceases in spring–summer; therefore, a pressure to restore an earlier (phase of) human activity in summer emerges associated with the advance of the sunrise times," wrote Martín-Olalla and Mira.
They found that human sleep-wake cycles tend to synchronize with winter sunrise times across locations, with rise times occurring approximately 1 hour prior to sunrise and bedtimes about 8.5 hours prior to sunrise. "Below the polar circles, the phase or timing of the sleep–wake cycle is synchronized in winter by the sunrise time (light) and not by solar noon (clocks)," they noted.
Regarding health risks associated with DST transitions, the investigators acknowledged the evidence of short-term effects, including increases in traffic accidents and myocardial infarction. However, they concluded that the effects were relatively minor: "The short-term increase of risks in accidents or in myocardial infarction is slight (z-score equal to 1/3) compared with the myriads of confounding factors."
They also addressed the concept of chronic misalignment with natural light cycles during DST, noting that such concerns may be overstated. If misalignment were a significant problem, individuals would have adjusted their schedules to compensate. "The regulations would have not sustained in time or [individuals] would have acted by delaying their preset time schedules in summer to offset the 'misalignment,' " they wrote.
Early April and early October may be optimal transition dates and align with current practices in Australia and New Zealand to minimize activity occurring in darkness.
"We do not support clock regulations to continue globally. We do not support that clock regulations must come to an end globally either," they noted.
The authors declared no competing interests.