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About that extra hour: how Daylight Saving Time affects our behaviour

30th Oct 2017

For those of us who have ever had a bad night’s sleep, it probably won’t come as a surprise that how well rested we are affects our behaviour. In Scarcity, for example, reductions in cognitive power are put in the context of a lost night of sleep. But what about the impact of much smaller changes to our sleep patterns?

This weekend, as clocks went back in the UK, many of us will have taken the opportunity for an extra hour in bed. And every year, social scientists watch as a huge natural experiment on the impact of sleep on behaviour unfolds.

Today, for example, you are more likely to be a victim of assault if you live in the UK. Far from a lack of sleep making us irritable and aggressive, a recent paper by academics at the University of Pennsylvania found that assaults are 2.8% higher the Monday after the clocks go back, compared to the following Monday.

The effect is almost perfectly mirrored in reverse, with a lost hour when the clocks go forward leading to a 2.9% decline in assaults. We might be irritable, but it seems that we can’t be bothered to act on it if we haven’t had a good night’s rest.

Not every effect is symmetrical though, and often a lack of sleep will have more impact than the hour gained. Several papers have found deaths due to traffic accidents rise the Monday after the clocks shift forward, but the findings for changes after the clocks go back are less consistent (they might even increase on the Sunday when we’re awake for longer than usual).

And while heart attacks significantly increase for the first three weekdays after the clocks move forward, the decline when the clocks move back is only significant for one day.

But before we start looking too harshly on Daylight Saving Time as a sleep-disrupting and violence-inducing system , it is worth noting that these effects only last for a few days. Over the course of summer we may have reaped the benefits, with fatal collisions with pedestrians reduced as a result of the better light in the busy evening hours. Others cite potential power savings and an end to the twice-yearly disruption to our clocks as reasons to use it all year round.