A lot of people dread the clocks going back an hour in winter – but reassure themselves that at least they’ll get an extra hour’s sleep. However, in my new study my colleagues and I found most people do not (or can not) take advantage of the full extra hour of sleep in autumn.
Daylight saving time is the practice of moving the clocks one hour forward in spring and one hour back in autumn. It was introduced during the first world war as a way to cut energy costs. It is in operation in around 70 countries and affects a quarter of the world’s population.
This “springing forward” and “falling back” is widely thought of a loss of one hour of sleep in spring and a gain of one hour of sleep in autumn. However, research suggests we may lose sleep for about a week after both clock changes as we struggle to adapt to the new time.
Previous studies have relied on people reporting their own sleep patterns in diaries or surveys. However, this may not be accurate because people sometimes forget or lie about how long they slept for. Recent research has overcome this problem by using activity monitors to record people’s sleep over the clock changes. But until now researchers have only been able to do this in a small number of people.
Our new study explored the effects of the clock changes on objectively-measured sleep duration in a large number of people who are signed up to the UK Biobank. This is a research database with lifestyle and health information from half a million UK participants. We analysed sleep data from 11,800 people who wore activity monitors for one or more days during the two weeks surrounding the spring and autumn clock changes in 2013-2015.
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We found that people slept for just over half an hour more on the Sunday of the autumn clock change than the surrounding Sundays. But people slept for around an hour less of the Sunday of the spring clock change.
Previous research suggests people sleep for less on the weekdays immediately after the clock changes than the weekdays before. In contrast, this study found that, overall, people were catching up on sleep on the Monday to Friday after both clock changes. This trend was stronger in spring after people had lost an hour of sleep. On average people slept seven minutes more per weeknight after the spring clock change and three minutes more per weeknight after the autumn clock change than the previous week.
This suggests that effects of the clock changes on sleep duration are more short lived than earlier studies reported. However, when we broke the data down, we found that this pattern of catching up on sleep was not seen in women. In fact, women often slept for less on the weekdays after the clock changes than before. This could be because women experience higher levels of insomnia and sleep difficulties and that these problems are exacerbated by the clock changes. Women are thought to struggle more with insomnia than men due to a number of reasons, including hormonal fluctuations, societal factors and higher rates of depression and anxiety.
We also found that, in autumn, older people and the retired slept less on the weekdays after the transition than before. It may be that older people are particularly vulnerable to their sleep being disrupted by the clock changes because sleep becomes lighter and more fragmented as we age.
Why does this matter?
Although short lived, the sleep loss seen over the spring clock change in our study has consequences for health, as just one night of bad sleep has been associated with a decline in mental and physical health.
Research has found that the clock changes themselves are associated with an increase in heart attacks, strokes, traffic accidents and depression. Sleep plays a vital role keeping your heart healthy, as well as maintaining emotional regulation. The amount of sleep you get also affects your reaction times and how likely you are to take risks.
Concern over the harmful effects of the clock changes on health has prompted sleep scientists to call for the clock changes to be abolished. Indeed, a growing number of countries – including the US, Jordan, Mexico, Ukraine and those in the EU – have made plans to do just that.
But stopping the clock changes is not straightforward. Plans in both the US and EU have stalled, with disagreements over what time to adopt permanently. Sleep experts argue that staying on winter (standard) time is best for health as this prioritises morning light which helps wake you up, resets your biological rhythm each day and makes it easier for you to fall asleep in the evening. Meanwhile, politicians are campaigning for permanent summer time due to the economic benefits they think it has.
The UK finds itself in an interesting position. No longer part of the EU, it is not duty bound to stop the clock changes at the same time as the EU. But being out of sync with the rest of Europe (including the Republic of Ireland) could have economic and logistical implications.
The UK government will probably review its daylight saving time policy as and when the EU finally ends the clock changes. It is crucial that they take the effects on sleep and health into account when this happens.
Melanie de Lange receives funding from Wellcome.
