Daylight Saving Time Explained: How Spring Forward Disrupts Your Biological Clock

What is Daylight Saving Time?

Daylight saving time (DST) is the practice of advancing clocks by one hour during warmer months to make better use of natural daylight. This spring forward transition, which occurred on March 29, 2026, may seem like a minor inconvenience, but health experts warn it significantly disrupts our circadian rhythm—the internal 24-hour biological clock that regulates sleep, alertness, mood, and hormone production. The circadian rhythm disruption caused by this one-hour shift can have profound effects on physical and mental health that last for weeks.

The Science Behind Circadian Rhythm Disruption

Our circadian rhythm is controlled by the suprachiasmatic nucleus (SCN) in the brain, which synchronizes with light cues to regulate physiological processes. When we spring forward for daylight saving time, this delicate system is thrown off balance. 'The most people are able to adjust within a week,' says Dr. Jeffrey Kelu, a circadian rhythm researcher at King's College London. 'But we also know there are people who have much difficulty adjusting their biological clock, and take weeks or even months.' According to Kelu, whose research focuses on circadian clock regulation of skeletal muscle growth, the internal biological clock is genetically determined, making some individuals more vulnerable to disruption.

Immediate Health Impacts

Research shows concerning health impacts following the spring transition. A 2024 analysis of 14 million US deaths found slight increases in all deaths after spring transitions, with dementia-related mortality peaking at 5% higher in the fifth week. The American Heart Association notes upticks in heart attacks on the Monday after DST begins and strokes for two days afterward. Driving safety is also affected, with drivers showing 13% more lane swaying and fatigue lasting up to a month.

Why Spring Forward is More Harmful Than Fall Back

The body's natural circadian rhythm (24.2-24.5 hours) makes advancing clocks more disruptive than delaying them, as it conflicts with our natural tendency to drift forward each day. This explains why the spring transition causes:

• Increased wakefulness during sleep (10-30 minutes more)
• Greater sleep fragmentation (5-20%)
• Poorer sleep quality overall
• Diet plays a role - people on high-fat diets experience prolonged circadian misalignment

The sleep disorders research community has documented these effects extensively, noting that night owls can require 2-3 weeks to realign their sleep-wake cycles compared to the 3-7 days most people need.

The Melatonin Factor and Light Exposure

One of the arguments for daylight saving time is that the sun shines longer in the evening. However, according to Kelu, this extended exposure to light late in the evening isn't good for humans. 'This is because the body no longer recognizes that it needs to relax and prepare to fall asleep,' he explains. Light inhibits the production of melatonin, the hormone produced by the pineal gland when the body prepares for sleep. The body produces the most melatonin in darkness, and production is suppressed when exposed to daylight.

Kelu recommends darkening the bedroom before bedtime to improve sleep quality. 'The body then gets the signal that it is no longer day.' Morning light is crucial for resetting our internal clocks, yet with DST, many people leave for school or work before seeing natural light during much of the year.

Long-Term Health Consequences

Circadian rhythms are essential for the body. When the biological clock is properly aligned with the day's rhythm, clear signals are sent for important bodily functions. People with stronger biorhythms tend to have more regular sleep schedules. Disruption of the biological clock—which can also occur through certain eating habits, jet lag, or irregular shifts—is associated with various health problems including:

A Stanford Medicine study published in September 2025 found that eliminating daylight saving time shifts would significantly improve Americans' health, resulting in approximately 300,000 fewer stroke cases and 2.6 million fewer obesity cases nationwide.

The EU Debate and Future of Time Changes

Each year, the debate flares up about whether to abolish daylight saving time. The European Commission proposed in 2018 to end seasonal time changes. Of the 4.5 million respondents, 84% supported abolition. Only in Cyprus and Greece is there a small majority for maintaining the current system. However, EU member states failed to reach agreement, and the proposal stalled in the European Council.

Spanish Prime Minister Pedro Sánchez is leading a renewed push to end daylight saving time changes across the European Union, declaring 'I don't see the point anymore' and planning to bring the proposal to the European Council. With the current EU time-change schedule expiring in 2026, there's renewed opportunity for action.

Which Time Should We Keep?

If clock changes are stopped, what's the best time to maintain? Kelu advises on behalf of other scientists to keep standard time—winter time. If daylight saving time becomes the permanent norm, most people wouldn't see light before heading to school or work for much of the year. 'Morning light is crucial,' says Kelu. 'We're stuck in offices and constantly exposed to artificial light.' Artificial light also plays a role in synchronizing the biological clock, but its effect is less powerful than natural light. 'We shouldn't deny ourselves that small amount of daylight.'

Frequently Asked Questions

How long does it take to adjust to daylight saving time?

Most people take 3-7 days to adjust, but night owls can require 2-3 weeks to realign their sleep-wake cycles completely.

What are the most serious health risks of spring forward?

Increased risks of heart attacks, strokes, fatal car accidents, and temporary spikes in all-cause mortality, particularly affecting those with pre-existing health conditions.

Why is spring forward worse than fall back?

The body's natural circadian rhythm (24.2-24.5 hours) makes advancing clocks more disruptive than delaying them, as it conflicts with our natural tendency to drift forward each day.

What can I do to ease the transition?

Get early morning sunshine to reset your internal clock, move up daily routines like dinner and exercise times gradually, and darken your bedroom before bedtime to improve sleep quality.

Will daylight saving time ever be abolished?

The EU continues to debate ending seasonal clock changes, with Spain recently restarting efforts. While public support is strong (84% in 2018 EU consultation), political and geographic complexities have stalled implementation.

Sources

RTE Brainstorm: Clock Change Health Effects
US News Health Report
Stanford Medicine Study
Dr. Jeffrey Kelu Research Profile
EU Daylight Saving Time Debate