The analysis

Solar time or summer time? The answers between energy saving and people's well-being

From the quality of 'sleep' to biological rhythms, here is what emerges from forty years of research on time change in a review conducted by the Centre for Sleep Medicine at Irccs Neuromed

by Andrea Romigi *

24 October 2025

3' min read

Translated by AI

Versione italiana

3' min read

Translated by AI

Versione italiana

Daylight saving time was introduced at the beginning of the 20th century with a seemingly simple objective: to save energy by making better use of daylight. Since then, it has become customary in over seventy countries, adopted in the name of efficiency and collective well-being. Yet, as time has passed, the focus has increasingly shifted from economic to health aspects, and today there is debate as to whether this shifting of the hands does not, at least for some, bring more disadvantages than benefits.

The Studio

Our review, conducted by the Centre for Sleep Medicine at Irccs Neuromed in collaboration with the University of Pavia, the Mondino Foundation, the National Research Council, the Uninettuno University and the University of Genoa, and published in the journal Sleep Medicine Reviews, brought together and critically analysed studies conducted over the last forty years on the impact of summer time. The aim was to assess whether seasonal transitions (particularly the spring one, when the hands move forward) really do affect sleep, alertness and health in general.

The effects

A clear trend emerges from an examination of the twenty-seven papers included. The switch to summer time leads to a reduction in sleep duration and an increase in its fragmentation. This results in greater daytime sleepiness and, in some cases, a decline in attention and concentration. The effects are most evident in evening chronotypes, the so-called 'owls', people who tend to fall asleep and wake up later, but also in adolescents and shift workers, categories that are already more vulnerable to alterations in the circadian rhythm.

The scientific literature also reports an increase in cardiovascular events and traffic accidents in the weeks following the spring transition. These are not dramatic effects in absolute terms, but signs consistent with the idea that our organism struggles to synchronise with an artificial time. The misalignment between the internal biological clock, the solar clock and the social clock, in fact, does not end in the days immediately following the change: it can continue for weeks, sometimes months, affecting sleep quality and hormonal regulation.

Staying in daylight saving time, i.e. adopting a time shifted forward throughout the year, does not seem to be a better solution either. Evidence suggests that this condition maintains a constant misalignment between natural light-dark cycles and our daily habits, with possible repercussions on metabolism, mood and cognitive performance. Daylight saving time, on the other hand, appears to be more physiological and consistent with human circadian rhythms.

Let's continue studying

The picture drawn by the available research, although heterogeneous in terms of methods and samples, is fairly solid. Seasonal changes, especially spring, have measurable effects on sleep and alertness. However, the lack of studies based on the use of polysomnography, the reference technique for analysing electrical sleep signals, indicates the need for more systematic and standardised research, which would allow a better understanding of the long-term impact of these shifts on people's biological and psychological balance.

Of course, there are also methodological limitations: many studies were conducted on small samples or with very heterogeneous assessment tools, which makes it complex to quantify the real effect of summer time in numerical terms. However, the consistency of the observations provides an important indication, which deserves to be taken into account in decision-making processes.

Modest savings

Lastly, there remains the economic question, which is at the very origin of summer time. The most recent analyses show that the energy savings today are very modest: they rarely exceed 0.5% of annual consumption and, in some regions, can even turn into an increase in costs, due to the prolonged use of air conditioning systems. It is therefore legitimate to wonder whether such a marginal advantage can justify the effects, albeit slight, that changing the time entails on health and well-being.

A review of the scientific evidence to date provides no reason to maintain daylight saving time, let alone make it permanent. On the contrary, it suggests that daylight saving time is the condition most consistent with our biological rhythms. Time-related policies, often regarded as a technical or administrative matter, should instead take human physiology into account: an extra hour of evening light may seem an advantage, but it is not always in harmony with our nature.

* Irccs Neuromed