The One-Hour Shift: What happens to your sleep, mood and focus when the clocks change?
Researched and written by Anamika Singh, Gwendolyn Vieson and Dr Helge Gillmeister for RESTED, October 2025
Why do the clocks change twice a year, and what do we know about the effects of shifting external time on our internal body clocks? Are some people more affected than others, and does it matter whether clocks go forward or back? How do researchers even measure these changes? This article reviews recent studies on the effects of Daylight Saving Time (DST) on health, sleep, mood and cognition, including the underlying mechanisms and individual differences such as your chronotype (whether you are more of a “morning lark” or a “night owl”). Research indicates that even a one-hour shift takes several days to adjust to, can disrupt neural and psychological processes, influencing sleep quality, mood regulation, and cognitive efficiency.
British Summer Time (BST) was originally established by the Summer Time Act of 1916, meaning that clocks in the UK have been adjusted twice annually for over a century. This involves advancing clocks by one hour at 1 a.m. on the last Sunday in March for British Summer Time (BST) and setting them back by one hour at 2 a.m. on the last Sunday in October to return to Greenwich Mean Time (GMT). This “Daylight Saving Time” (DST) is intended to make better use of natural daylight during the longer days of summer and to reduce energy consumption. The relevance of these clock changes remains widely debated, with many regions around the world opting out in recent years. For countries such as the UK, which continue to observe the time changes, understanding their potential effects on people is of growing importance.
People’s mood, cognition, sleep and health are systematically impacted by clock changes. This has been documented in numerous studies, using both experimental and observational methods. Some studies employ cognitive tasks and sleep tracking to measure changes in attention and reaction time, or activity cycles and sleep quality, others look at more subjective measures like self-reported mood or sleep diaries, while yet others analyse large-scale population-level data such as hospital records (e.g., Johnson & Malow, 2022; de Lange et al., 2025).
A review article (summarising the findings from many studies) by Johnson and Malow (2022) reported that disruptions to the circadian rhythm – the bodily fluctuations that repeat every 24 hours – from DST can trigger or worsen mood disorders like depression. Many antidepressant medications act on the circadian system, either delaying or advancing its phases (e.g., fluoxetine causes phase advances, lithium causes phase delays). Consequently, when circadian stability is further disturbed by clock changes, rates of depression, suicide, seasonal affective disorder and substance misuse may rise.
Berk et al. (2008) suggest that when there are sudden changes in external environmental cues, it desynchronises our internal circadian rhythm, causing a rise in adverse outcomes like suicides. Others suggest that when we change to an earlier onset of sundown (in October), these light cues mark the start of a long period of shorter days. This can signal distress in some people (Hansen et al., 2015), causing the rise in mood disorders. Hansen’s study measured hospital contacts for unipolar depressive episodes using data from a Danish nationwide psychiatric register spanning 17 years of transitions from summer time to standard time.
However, a more recent study looking at the GP and hospital-admission records of over 600,000 UK residents, found that there was actually a reduction in reported health events related to anxiety, depression, psychiatric and sleep disorders in the weeks following the October clock change, and no effect on any these following the March clock change (de Lange et al., 2025). This suggests that people may actually benefit from delays (when the clocks go back) (see also Kantermann et al., 2007).
Sleep itself becomes more fragmented after the transition to and from BST, but slightly more so when going from to BST to GMT (in October in UK) than from GMT to BST (in March). This was shown in a study by Lahti et al. (2008) using wrist-worn accelerometers (measuring movements) in nine healthy 20-40 year-olds.
Reductions in sleep quality can make you feel more sleepy or drowsy during the daytime. contributing to diminished alertness and cognitive performance. Indeed, slowed reaction times on a vigilance task have been shown in adolescents after shifting into DST (in March in the UK) in a study of 35 US high schoolers (grades 9-12) using accelerometers, sleep diaries and cognitive tests (Medina et al., 2015). Accelerometer analyses showed that these adolescents cumulatively lost almost 3 hours of sleep in the week after the clock change compared to the week before. Sleep diaries showed an average of 27 minutes of sleep lost per night. Unsurprisingly, there was an increase in reported daytime sleepiness, too. Cognitive tests showed slowed responding and more frequent lapses in attention. Being more sleepy and less vigilant can make you more prone to accidents and have poorer academic performance. It also raises concerns for drivers. For example, an increased incidence of motor vehicle collisions has been documented in people living in eccentric timezones, where clock time misaligns with people’s rest-activity cycles (this is called “social jetlag”) (Gentry et al., 2022).
Finally, transitions between BST and GMT have also been associated with changes in neurological health outcomes. For example, within days of each clock change, there is a measurable increase in hospital admissions for ischaemic strokes (Sipilä et al., 2016). As the cardiovascular and nervous systems are regulated by circadian control, it is thought that a shift in the circadian rhythm may increase people’s susceptibility to neurological events such as strokes and heart attacks (Čulić & Kantermann, 2021; but see de Lange et al., 2025 for a reduction in cardiovascular disease events when changing from BST to GMT specifically).
Not everyone responds to clock adjustments in the same way. Research indicates that an individual’s chronotype, whether they are more of a morning person (“morning larks”) or an evening person (“night owls”), can influence how quickly they adjust to DST transitions. Evening chronotypes often taking longer to fully adjust to or from a DST transition (Xu et al., 2024). Evening chronotypes also tend to experience more adverse effects following the transition from GMT to BST (in March in the UK), while morning chronotypes find BST-GMT transitions more disruptive (in October in the UK) (Lahti et al., 2008). While there are individual differences, most people adjust more readily to delays (in October) than to advances (in March), which is also seen when measuring the effects of jetlag from travelling westward compared to eastward (see Kantermann et al., 2007).
A quarter of the world’s population experiences clock changes twice every year. To mitigate potentially adverse outcomes, you might consider gradual behavioural adjustments prior to clock changes, especially at the start of DST (in March in the UK). Increasing exposure to morning light may help stabilise circadian rhythms and enhance alertness and mood. Similarly, advancing or delaying sleep schedules incrementally in the days preceding each change can facilitate smoother adaptation and reduce physiological strain.
References:
Berk, M., Dodd, S., Hallam, K., Berk, L., Gleeson, J., & Henry, M. (2008). Small shifts in diurnal rhythms are associated with an increase in suicide: The effect of daylight saving. Sleep and Biological Rhythms, 6(1), 22-25. https://doi.org/10.1111/j.1479-8425.2007.00331.x
Čulić, V., & Kantermann, T. (2021). Acute myocardial infarction and daylight saving time transitions: is there a risk?. Clocks & sleep, 3(4), 547-557. https://doi.org/10.3390/clockssleep3040039
de Lange, M. A., Birnie, K., Richmond, R. C., Shapland, C. Y., Eastwood, S. V., Tilling, K., & Davies, N. M. (2025). The Effects of Daylight Saving Time Clock Changes on Mental and Physical Health in England: Evidence from the Clinical Practice Research Datalink (CPRD). medRxiv, 2025-05. https://www.medrxiv.org/content/10.1101/2025.05.14.25327580v1
Gentry, J., Evaniuck, J., Suriyamongkol, T., & Mali, I. (2022). Living in the wrong time zone: Elevated risk of traffic fatalities in eccentric time localities. Time & Society, 31(4), 457-479. https://doi.org/10.1177/0961463X221104675
Hansen, B. T., Sønderskov, K. M., Hageman, I., Dinesen, P. T., & Østergaard, S. D. (2017). Daylight savings time transitions and the incidence rate of unipolar depressive episodes. Epidemiology, 28(3), 346–353. https://doi.org/10.1097/ede.0000000000000580
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Lahti, T. A., Leppämäki, S., Lönnqvist, J., & Partonen, T. (2008). Transitions into and out of daylight saving time compromise sleep and the rest-activity cycles. BMC Physiology, 8(1). https://doi.org/10.1186/1472-6793-8-3
Medina, D., Ebben, M., Milrad, S., Atkinson, B., & Krieger, A. C. (2015). Adverse effects of Daylight Saving Time on adolescents’ sleep and vigilance. Journal of Clinical Sleep Medicine, 11(08), 879–884. https://doi.org/10.5664/jcsm.4938
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