Commentary: How the March clock change affects sleep, health and daily rhythms

Are you worried about losing sleep when we switch to Daylight Saving Time on 29th March?

Across Europe, we will shift our clocks by one hour. This happens overnight so that, early on Sunday morning, the clocks jump from 12:59 to 02:00 instead of progressing in the normal way from 12:59 to 01:00. This vanishing hour means that pets and young children will appear to “sleep in” on Sunday morning, even though they sleep no more hours than usual. If we ourselves have forgotten about the changing clocks, we may also be surprised how “late” we wake, particularly now that our phones and digital watches automatically adjust.

This jump in time has a ripple effect. On Sunday evening, we are not ready to go to sleep when our clocks proclaim it is our usual bedtime, exacerbating the commonly experienced Sunday night poor sleep effect, with shorter sleep on Sunday night. Studies report a small increased risk of cardiac events (specifically acute myocardial infarction) in the days following the shift to Daylight Saving Time (DST) and, in some studies, more fatigue-related traffic accidents than usual.

These are useful reminders of the importance of sleep. We know we need sleep to function effectively and for long-term health and wellbeing. Yet we are regularly deprived of sufficient sleep as we juggle family, work and social life. We regularly shift the timing of our sleep by travelling across time zones – or simply taking a holiday at home and letting our bed and wake times drift.

The difference with the shift to DST is that these individual sleep disruption events are synchronised. It is a countrywide one-hour jet lag effect. As found by researchers studying more than 24,000 users of a wrist-worn wearable in the USA, public holidays are other synchronisation events which, at a population level, result in significant transitory changes to sleep onset, offset and duration.

So, should we be worried about the effects of the shift? At the individual level, no one has yet quantified the risk associated with the DST transition as compared with the risk of returning to work from a holiday, or staying up unusually late from attending a social event or watching a sporting or cultural event on TV. If you are concerned, it is also possible to prepare, much as you could do when planning to travel across time zones, by shifting gradually ahead of time. Importantly, the effects of changing to DST on sleep and health outcomes are transient, and after a few days on the new time it feels like normal.

Should we continue to change the clocks? If we rarely go outside and live and work in environments in which there is little natural lighting, whether we are on standard time or DST makes no difference.

However, for healthy circadian function, we need bright light during the biological day, dim light before we go to bed and dark at night. Since light outdoors during the day is typically a hundred times brighter than light indoors, the cheapest way to get bright light is to access natural light either by going outside during daylight hours or sitting near a window. During the short winter days, when dawn is late and dusk is early, standard time is generally agreed to give us the best opportunity for sufficient bright light early enough in the day to support healthy circadian function.

However, by the time we get to Saturday 28th March, dawn in London will be 05:11 and dusk at 19:00. When the clocks change on the morning of 29th March, dawn and dusk both move one hour later. Since most of us get up after 06:11 and go to bed after 20:00, few of us will notice that the mornings have got darker, but many of us will notice that the evenings are lighter. Whether we are on standard time or DST, the opportunity for access to bright morning light is therefore much the same. Under DST, the evenings are brighter, but there is insufficient evidence to know whether the disruptive health effect of the evening bright light is counterbalanced by the increased opportunity for other leisure activities.