The circadian rhythm is like a conductor orchestrating an orchestra
Twice a year, societies perform a small act of collective disruption — moving the clock forward or back — and for generations this has been treated as a minor inconvenience. A new Stanford University study published in PNAS now places that ritual under a harder light, finding that these biannual shifts correlate with measurable increases in obesity and stroke risk by destabilizing the circadian rhythms that quietly govern our hormones, metabolism, and cardiovascular health. The researchers argue that the human body, much like a finely tuned orchestra, performs best under a steady conductor — and that permanently adopting standard time could spare thousands from preventable illness.
- Twice-yearly clock changes are not merely inconvenient — Stanford researchers have linked them to elevated stroke risk and higher obesity rates by repeatedly destabilizing the body's internal 24-hour rhythm.
- The spring change proves especially harmful: losing an hour of sleep in March creates more severe circadian misalignment than the autumn gain, exposing a hidden asymmetry in a ritual most people treat as equal in both directions.
- Across a population the size of the United States, the projected benefits of eliminating clock changes are striking — an estimated 0.78% reduction in obesity and 0.09% fewer strokes, translating to thousands of prevented cases annually.
- Researchers point to morning light as the body's primary calibration tool, and argue that permanent standard time aligns more naturally with human biology than daylight saving time, making it the preferable fixed option.
- Portugal faces its next clock change on October 26, and as health evidence accumulates, the political and public debate over whether to end the biannual cycle may be approaching a turning point.
Late October approaches, and with it the familiar ritual of setting clocks back. But a new Stanford University study, published in the Proceedings of the National Academy of Sciences, suggests this biannual habit is quietly exacting a measurable toll on public health — linking seasonal time changes to increased obesity and elevated stroke risk.
The mechanism lies in the circadian rhythm, the body's internal 24-hour clock that coordinates hormone release, digestion, metabolism, and sleep. Study co-author Jamie Zeitzer described this system as a conductor synchronizing an orchestra of organ systems. When the clock is stable and morning light exposure is consistent, the conductor leads with precision. When the clock is reset twice a year, that coordination degrades — and the body pays a physiological price.
The scale of potential benefit from ending clock changes is notable. Permanent standard time in the United States, the researchers calculated, could reduce obesity by 0.78% and stroke cases by 0.09% — modest percentages that represent thousands of prevented cases across a population of 330 million. The spring change emerged as the more damaging of the two shifts: losing an hour of sleep in March disrupts circadian alignment more severely than the autumn gain, though neither transition is without cost.
The study favors permanent standard time over permanent daylight saving time, reasoning that the human body evolved to receive more light in the morning and less at night — a pattern standard time better supports. Both fixed options, however, are superior to the current system of switching back and forth.
In Portugal, the next change arrives October 26. Whether the growing scientific case against clock-switching will translate into policy remains uncertain, but the evidence has rarely been more pointed.
Late October is coming, and with it comes the familiar ritual of turning clocks backward. But a new study from Stanford University suggests that this biannual shuffle—and the one that happens again in spring—may be quietly harming our health in measurable ways. Researchers publishing in the Proceedings of the National Academy of Sciences found that the twice-yearly time shifts correlate with increased obesity and elevated stroke risk, adding scientific weight to what many people have long sensed: that messing with the clock messes with the body.
The research focused on how these seasonal changes disrupt the circadian rhythm, the internal 24-hour clock that governs everything from hormone release to digestion to the basic rhythm of waking and sleeping. Jamie Zeitzer, one of the study's authors, was direct about the findings: staying on a single time standard year-round—whether standard or daylight saving—is measurably better than switching twice annually. The body, it turns out, does not adapt well to being reset twice a year, no matter how minor the shift seems.
What makes this finding significant is the scale of potential benefit. If the United States were to adopt permanent standard time, the researchers calculated that obesity rates could drop by 0.78 percent and stroke cases by 0.09 percent. Those numbers may sound small in isolation, but across a population of 330 million people, they represent thousands of prevented cases. The mechanism is straightforward: circadian disruption weakens the body's ability to regulate metabolism and cardiovascular function. When the clock is stable, these systems operate more efficiently.
Zeitzer used an apt metaphor to explain the stakes. The circadian rhythm, he said, functions like a conductor orchestrating an orchestra—synchronizing multiple organ systems so they work in concert. A stronger, more stable internal clock, reinforced by consistent morning light exposure, produces better-coordinated physiological function across the board. Conversely, when people experience repeated disruptions to their sleep-wake cycle, the conductor loses precision. The body receives light signals at the wrong times, weakening the overall rhythm.
The research also illuminates why spring's time change appears more harmful than fall's. When clocks spring forward in March, people lose an hour of sleep, and the disruption to circadian alignment is more severe. The fall change, which grants an extra hour in bed, produces less physiological stress—but it still interrupts the body's internal synchronization. Neither shift is ideal; both are suboptimal compared to maintaining a fixed schedule.
From a practical standpoint, the study suggests that permanent standard time would be preferable to permanent daylight saving time, with both being superior to the current system of switching back and forth. The reasoning centers on light exposure: the human body evolved to receive more light in the morning and less at night. Standard time aligns more closely with this natural pattern, allowing the circadian system to calibrate itself more accurately to the 24-hour day.
In Portugal, the next clock change arrives on October 26, when clocks will be set back one hour at 2 a.m., becoming 1 a.m. The last adjustment happened on March 30, when clocks moved forward. For now, the country continues the biannual cycle that research increasingly suggests is working against public health. Whether this evidence will prompt policy changes remains an open question, but the scientific case for stopping the clock-switching has never been clearer.
Citações Notáveis
Staying on a single time standard year-round is definitively better than changing twice per year— Jamie Zeitzer, Stanford researcher
The circadian clock is intimately involved in regulating human health, so disruptions lead to negative health consequences both directly and indirectly— Jamie Zeitzer, Stanford researcher
A Conversa do Hearth Outra perspectiva sobre a história
Why does a one-hour shift matter so much? Isn't the body flexible enough to adapt?
The body is flexible, but only within limits. The circadian rhythm is ancient—it evolved over millions of years to track the sun's actual position. A one-hour shift twice a year doesn't give the system time to adapt; it just keeps disrupting the synchronization that took weeks to establish.
So the problem isn't the change itself, but the repetition?
Exactly. If we changed once and stayed changed, the body would recalibrate within a few weeks. But we flip it back and forth, so the internal clock never settles. It's like trying to sleep while someone keeps nudging you awake.
The study mentions morning light exposure. Why is that so critical?
Light is the primary signal that tells your circadian system what time it actually is. When you get bright light in the morning, it anchors your entire 24-hour rhythm. Permanent standard time means more morning light naturally aligns with when people are typically awake, so the system stays stronger.
If the health gains are real—fewer strokes, less obesity—why haven't countries already stopped doing this?
Inertia, mostly. The practice is over a century old. There's also disagreement about whether people prefer more evening light in summer, even if it costs them health. But this research is the kind of evidence that eventually shifts policy.
Does the study say anything about who's most at risk from these changes?
Not specifically, but people with existing cardiovascular issues or metabolic disorders are likely more vulnerable. Anyone whose circadian system is already fragile would suffer more from the disruption.