Dr Eva Winnebeck

Earlier human activity relative to sunrise and sunset, the very essence of DST, is linked with health and safety detriments in humans. A new study predicts that deer, at least, may benefit from earlier human activity through reduced deer-vehicle collisions. Daylight saving time (DST) regulations are under debate again in many countries around the world. The US and the EU are considering abolishing the biannual switches into and out of DST but have yet to decide if they will settle for permanent Standard Time or permanent DST. The public and political debates are ripe with personal anecdotes, wrong assumptions and strong convictions, so empirical evidence on the advantages of either timing system are all the more important to encourage evidence-based policy – and highlight unintended side effects that may require tailored mitigation measures. In this issue, Cunningham et al. 1 investigated one of these potential side effects by analyzing the likelihood of deer-vehicle collisions in the US under different timing regimes, finding that the timing of humans on the road to and from work significantly adds to the risks of wildlife encounters and collisions. Chronobiology, the study of biological rhythms, has long recognized that, for each animal, there is a time and a place to be active, rest, eat or reproduce. This timing is not random but heavily influenced by endogenous timing mechanisms that interpret and synchronize to Earth's timing signals, like day and night and the seasons. Deer, as crepuscular animals, lock their activity to the hours before and after sunrise and sunset, which means that the clock times at which they are most active changes throughout the year together with the increase and decrease in daylength over the seasons. Their seasonal program will also lock their breeding to fall, in white-tailed deer in North America even to a very narrow window of 2-3 weeks 2,3 (Fig. 1). In contrast, humans are day active mammals, who have introduced artificial conditions into their lives that reduce the connection with natural light to the detriment of their daily and seasonal rhythms 4. We shield from the sun by hiding indoors, turn on light when it is dark outside and live to rigid work schedules that do not reflect local solar time but are the same across an entire time zone-often too wide or ill-placed, ignoring the fact that even under these conditions our biology will seek to respond to the sun 4-8. DST is another factor in this chronobiological problem humans are facing, shifting around the timing of entire societies twice a year or even permanently as currently under proposition. DST brings human activity forward by one hour by moving clocks and thus schedules one hour earlier. This leads to an abrupt advance of human activity in relation to the timing of sunlight (Fig. 1), with acute health and safety detriments 9-12. The long-term effects are much harder to quantify. However, theory and indirect data clearly indicate that what might still resonate with our biology during midsummer when daylight