Daylight Saving Time: Why the 1-Hour Shift Impacts Your Sleep & Health

Analysis: It may be a simple one hour shift, but it can cause some of us to struggle with fatigue, irritability and brain fog for a few days or even weeks

By Stefano Arlaud, Queen Mary University of London

The spring clock change, a twice-yearly ritual affecting roughly a quarter of the world’s population, isn’t merely a minor adjustment. Although seemingly simple, resetting clocks forward can trigger a cascade of physiological and psychological effects, leaving many grappling with fatigue, irritability, and a persistent mental fog. For some, particularly those who identify as “night owls,” this period of readjustment can extend for weeks. Understanding why the clocks going forward affects you longer than you realise requires a look into the science of circadian rhythms and how our bodies respond to disruptions in our internal clocks.

Circadian rhythms, the natural, internal processes that regulate sleep-wake cycles and other physiological functions, are present in plants, animals, and even bacteria. This widespread biological phenomenon underscores life’s inherent sensitivity to environmental cues, particularly light. Your body’s master clock, located in a region of the brain called the hypothalamus, governs hormone release, body temperature, and metabolism. When this rhythm is disrupted, these essential processes fall out of sync, leading to the unpleasant symptoms many experience after the clock change.

Most people require three to seven days to adjust to daylight saving time, but research indicates that night owls can accept two to three weeks to realign their sleep-wake cycles. This prolonged adjustment period is linked to the fact that our internal clocks naturally run slightly longer than 24 hours – typically between 24.2, and 24.5 hours. Delaying sleep, as happens with the spring transition, is more disruptive than advancing it, because it clashes with our body’s natural tendency to drift forward each day.

How Does Diet Play a Role?

Interestingly, research suggests that diet can influence the severity and duration of circadian misalignment. Studies have shown that individuals consuming high-fat diets may experience a more prolonged adjustment period after the spring clock change. A 2008 study on rodents, for example, found that those fed a high-fat diet adapted 20% more slowly to a six-hour light shift compared to those on a low-fat diet. While the precise mechanisms underlying this connection remain unclear, scientists are exploring potential links between dietary fat intake and circadian sensitivity to light.

Exposure to natural light is a critical factor in facilitating adaptation to time changes. The more morning sunlight a person receives, the faster their circadian rhythm realigns. Research suggests that adaptation is generally harder in spring than in autumn, with increased wakefulness during sleep, sleep fragmentation, and poorer sleep quality observed after the spring transition.

Beyond Sleep: Wider Health Implications

The impact of daylight saving time extends beyond mere sleep disruption. Research has begun to explore potential links between the clock change and increased health risks. Studies have indicated a slight increase in all-cause mortality following the spring transition, particularly related to cardiovascular complications. A 2024 study analysing 14 million deaths in the US from 2015 to 2019 found a rising trend in dementia-related mortality, with a 5% increase in deaths peaking in the fifth week after the spring transition. A slight increase in cancer-related mortality was also noted in the first week after daylight saving time begins.

The spring clock change also appears to have implications for road safety. A 2023 study investigating driving fatigue found that drivers exhibited signs of greater fatigue after the clock change, with increased lane swaying and eyelid closures. Notably, these impairments persisted for up to a month after the transition.

The Science Behind the Disruption

Our core body temperature naturally increases throughout the day, peaking in the late afternoon. Research has shown that the spring transition can delay this temperature peak by 30 to 60 minutes, further contributing to circadian misalignment. This disruption is reflected in measurable changes in sleep quality, with individuals experiencing more fragmented sleep and increased nighttime movement after the clock change.

The human circadian rhythm’s natural tendency to run slightly longer than 24 hours explains why delaying sleep (spring transition) is more challenging than advancing it (autumn transition). The spring transition disrupts melatonin release, a hormone crucial for regulating energy levels and sleep onset. German researchers monitoring 50 healthy adults found that spring adaptation took five to seven days longer than fall adaptation.

What Can You Do?

While the debate over abolishing daylight saving time continues, individuals can take steps to mitigate its disruptive effects. Prioritizing exposure to morning sunlight, maintaining a consistent sleep schedule, and adopting a healthy diet low in fat can all help to facilitate circadian realignment.

The spring clock change is more than just a minor inconvenience; it’s a disruption of our fundamental biological rhythms with potentially far-reaching consequences. Understanding these effects is crucial for navigating this twice-yearly transition and protecting our health and well-being.

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Stefano Arlaud is a PhD candidate in Time Processing and Metacognition of Time Processing at Queen Mary University of London. This article was originally published by The Conversation.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ