Twice a year, around a quarter of the world's population dutifully reset their clocks. It may seem like a minor adjustment, but some people struggle with fatigue, irritability, and brain fog in the days following the transition. For others - especially night owls - the adjustment period can last for weeks.
Author
- Stefano Arlaud
PhD candidate in Time Processing and Metacognition of Time Processing, SBBS, Queen Mary University of London, Queen Mary University of London
Circadian rhythms govern many physiological processes in plants , animals and even bacteria , highlighting life's remarkable sensitivity to changes in environmental conditions.
Your biological internal clock is controlled in a small region of the brain called the hypothalamus . It regulates hormone release, body temperature and metabolism. So if your circadian rhythm is out of kilter, those things will be disrupted too.
Most people take three to seven days to adjust to daylight saving time (DST). However, night owls can take two to three weeks to realign their sleep-wake cycles.
Research suggests diet also plays a role. People who eat high-fat diets seem to experience prolonged circadian misalignment after the spring clock change. A 2008 study on rodents found that those on high-fat diets adapted 20% more slowly to a six-hour light shift compared to those on low-fat diets. Scientists don't fully understand why diet and circadian rhythms are linked.
We do know light exposure is also important for adapting to time change. One hypothesis suggests that a high-fat diet reduces circadian sensitivity to light. Researchers have wondered whether the connection between high-fat diet and circadian sensitivity may be since late evening eating is associated with weight gain. But a 2024 study found no significant differences in meal timing between diet groups, suggesting that it's the food itself, rather than the time it is eaten, that's the key factor.
Exposure to natural light is one of the most important factors in helping the body adjust to a new time. The more morning sunlight a person gets, the faster their circadian rhythm realigns. Research suggests that adaptation is harder in spring than in autumn, with increased wakefulness during sleep (10-30 minutes more), greater sleep fragmentation (between 5-20%), and poorer sleep quality after the spring transition.
The link between natural light and cognitive function was highlighted by a 2020 study which demonstrated the benefits of increased daylight exposure. Thirty participants spent one week working in each of two office environments with identical layouts, furnishings and orientations. But one was fitted with smart glass (that can change its tint) and was set to optimise daylight. And the other had traditional blinds, that were closed. Participants in the optimised daylight condition slept 37 minutes longer and scored 42% higher on decision-making tasks.
The human circadian rhythm runs slightly longer than 24 hours (typically 24.2-24.5 hours). This makes clock delays (autumn transition) easier to adjust to than clock advances (spring transition) because our body naturally drifts forward each day. Delaying sleep aligns with this tendency, whereas advancing sleep disrupts melatonin release, which regulates your energy levels and the natural urge to go to bed.
In 2007, German researchers monitored 50 healthy adults for four weeks before and after each transition and found that spring adaptation took five to seven days longer than fall adaptation.
Our core body temperature increases throughout the day, peaking in the late afternoon. A 2008 Finnish study studied nine adults before and after both transitions and found that during the spring transition, people's gradual increase in body temperature was delayed by 30-60 minutes. It also found sleep quality dropped by 5-15%, and nighttime movement increased by 10-25% - all indicators of circadian misalignment . Total time in bed increased after the spring transition but participants' sleep was fragmented and of lower quality.
The spring clock change seems to create a slightly increased risk for those with life-threatening health conditions. Research has linked daylight saving time (DST) transitions to changes in mortality rates, during the first eight weeks after the transition, particularly in relation to cardiovascular complications. A 2024 study analysing 14 million deaths in the US from 2015 to 2019 found a slight increase in all deaths after the spring transition but a decrease in mortality after the autumn transition.
The study also found a rising trend in dementia-related mortality, with a 5% increase in deaths peaking in the fifth week after the spring transition. Additionally, a slight increase in cancer-related mortality was noted in the first week after DST begins.
Research also shows it's a good idea to pay extra attention when you're on the roads after the clocks go forward. A 2023 study investigating the effects of DST on driving fatigue found drivers showed signs of greater fatigue after the clock change. Their cars swayed in their lanes about 13% more often and their eyelids closed slightly more often. Participants still showed impairment one month later.
However, in a follow-on trial after the autumn return to standard time, drivers reported feeling less sleepy.
These findings suggest the spring transition can have a ripple effect that lasts for weeks. It also suggests we are more finely tuned to the natural world than we might think.
Spring DST may seem like a simple one-hour shift, but for many, it's much more than that.
Stefano Arlaud does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
