An Extra Hour Of Daylight In The Evening In Summer: Is It Good Or Bad?
Every year, the arrival of Spring brings a recurring discussion: should we advance our clocks so that daylight stays until later? This political decision results in an extra hour of light in the evening and affects a quarter of the world's population. But is this the best decision according to our physiology?
Every year, the setting of Spring comes with a recurrent discussion: should we change our clocks to daylight saving time (DST)? This political decision results in an apparent extra hour of light in the evening during the summer, and affects a quarter of the world’s population. But is it the best decision according to our physiology? Now,a new study led by Cátia Reis, a researcher at the Instituto de Medicina Molecular João Lobo Antunes (iMM, Portugal), and published in the scientific Journal of Pineal Research, shows that changing the clocks in the Spring costs more than an hour of sleep to patients suffering from a sleep disturbance called delayed sleep-wake phase disorder.
Most organisms respond to an internal clock that adapts the physiological responses to solar time. In mammals, the light and dark cycle is by far the most important stimulus. “Our body responds to the light and dark cycle by producing melatonin, which induces sleep. This hormone, often called the hormone of the night, is produced in response to the absence of light and degraded by light in the morning”, explains Cátia Reis, leader of the study, adding: “Usually this hormone is produced on average up to 3 hours before sleep. Each person has its own timing of melatonin production. Some people produce it earlier, and get tired sooner, but also wake up early in the morning: we call them “larks”. Others are more like owls, they stay up later at night and sleep into the day. Most of us are actually like doves, lying in between these extremes. People suffering from delayed sleep-wake phase disorder are extreme late owls”.
In this study, the researchers analyzed 82 patients from CENC, a sleep medicine center in Lisbon (GMT time zone), and recorded the sleeping timings and melatonin production timing of those coming to the clinic in standard or daylight saving time. “We found that during daylight saving time, patients would still fall asleep at a similar time, but would wake up more than an hour earlier on work days. This results in a sustained loss of sleep. The daylight saving time takes an hour of light in the morning, when we need light to degrade melatonin, and adds it to the night, when we need dark to produce melatonin”, says Cátia Reis, on the results of the study.
As for the timing of melatonin production, it is the same in standard or daylight saving time, specially when corrected for solar time, suggesting that the change only increases the distance between local and solar time. Since the melatonin production time is the same according to the solar time, people will produce it later by clock time, but due to social constrains they wake up at the same clock time, meaning one hour earlier. “This effect is more prominent in patients suffering from delayed sleep-wake phase disorder but can be extrapolated to the general population. Other researchers have found negative implications of this uncoupling between solar and biological time, such as an increased risk to develop certain diseases, or an increase in the frequency of accidents on the days following the change”, says Cátia Reis.
“Over the course of industrialization, the decrease in light during the day inside buildings, and the electrical lights at night, has changed most human clocks to become later: we are all turning into owls. But our social schedules are still rather early, and misaligned with our internal clocks that respond to the solar time. We have the sleep patterns of owls, but our social lives require us to act like larks! This misalignment is associated to health deficits. The change to the daylight saving time in the Spring only builds on this issue by taking an hour of light in the morning and changing it to the night, increasing the discrepancy between solar time and clock time”, adds Cátia Reis, taking a stance on this matter.
Cátia Reis, Luísa K. Pilz, Achim Kramer, Luísa V. Lopes, Teresa Paiva, Till Roenneberg. (2023). The impact of daylight-saving time (DST) on patients with delayed sleep-wake phase disorder (DSWPD). Journal of Pineal Research. DOI: 10.1111/jpi.12867