It's the dreaded time of year when we lose an hour's sleep as the clocks go forward.. It may seem a minor adjustment in return for the longer hours of daylight in summertime, but some of us can struggle for weeks.
Fatigue, irratibility and brain fog can last for days or even weeks following the clocks changing, warns one expert. Stefano Arlaud, researcher in time processing, warns the adjustment period can affect people more than they realise and isn't limited to finding it harder to get out of bed.
Circadian rhythms govern many physiological processes in humans, plants and even bacteria, highlighting sensitivity to changes in environmental conditions like getting up an hour early, he explained. In humans our biological internal clock is controlled in a small region of the brain called the hypothalamus. You can get more story updates straight to your inbox by subscribing to our newsletters here.
That part of our brains regulates hormone release, body temperature and metabolism. So if our 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," said Stefano, from London's Queen Mary University, "research suggests diet also plays a role - people who eat high-fat diets seem to experience prolonged circadian misalignment after the spring clock change."
Light exposure is also important for adapting to time change and it could be that a high-fat diet reduces circadian sensitivity to light, he said: "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."
Writing in The Conversation the expert explained that 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."
The spring clock change also seems to cause a slightly increased risk for people with some life-threatening health conditions. Research links daylight saving time (DST) changes 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. That study also detected 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.
And some studies also show 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 and 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 effects that lasts for weeks and that 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," warned Stefano.
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