Which Permanent Time Do Sleep Scientists Recommend?

If Spain stopped changing the clocks, one question would remain: which time should stay—summer time or winter time? For sleep scientists, the prevailing answer is permanent standard time, commonly called winter time. But Spain has an extra complication: its standard time is already an hour ahead of neighbouring Portugal’s. Understanding the recommendation means separating three things that often get tangled together: the clock change, the time zone and the schedules you live by.

The short answer: permanent standard time

Standard time is a country’s base clock setting, without the extra hour added for daylight saving. Daylight saving time, or summer time, moves the clock one hour ahead of that base setting.

For mainland Spain, the options would look like this:

UTC, short for Coordinated Universal Time, is the reference used to describe time zones. UTC+1 simply means the clock reads one hour ahead of that reference. The Canary Islands use clock settings one hour behind mainland Spain.

The European Sleep Research Society, the European Biological Rhythms Society and the American Academy of Sleep Medicine have recommended permanent standard time rather than permanent summer time. Their main concern is how clock time aligns everyday life with daylight.

The sleep-science recommendation is not “keep winter.” It is “choose the clock setting that better supports the body’s daily timing.”

Keeping winter time would not make the days shorter, remove summer evenings or change the seasons. It would change the numbers on the clock when daylight occurs.

Why morning light matters more than the clock label

Your body keeps its own time

Your circadian rhythm is the roughly 24-hour pattern that helps regulate sleepiness, alertness and other bodily functions. Think of it as an internal timetable—not perfectly accurate on its own, but regularly adjusted by cues from the outside world.

Light is one of the most important of those cues. In broad terms:

• Morning light helps set the body clock and supports an earlier sleep–wake rhythm.
• Light later in the evening can push that rhythm later, making you ready for sleep later.

The issue is therefore not simply whether you have enough daylight. It is when that daylight reaches you relative to when you must wake, work, study and sleep.

Summer time does not create more daylight

Moving the clocks forward changes the clock labels attached to sunrise and sunset. It does not change the amount of daylight.

Suppose, on a particular winter day, sunrise would occur at 08:30 under standard time. Under permanent summer time, that same sunrise would be called 09:30.

If school or work still begins at 09:00, the difference matters: under the summer-time setting, you may have to start your day before sunrise, with less opportunity for morning light.

At the other end of the day, daylight also appears an hour later on the clock. That can be appealing for outdoor activities, but later light can encourage later sleep timing. If your morning alarm remains fixed, a later bedtime may leave less room for sleep.

This is the central biological argument behind the European sleep and biological-rhythm societies’ recommendation: standard time generally aligns social schedules with the light–dark cycle better than summer time does.

How strong is the health evidence?

A recommendation can be well grounded without every long-term outcome being settled.

It helps to distinguish two different questions:

1. What happens when the clocks change?
   This concerns the short-term disruption caused by shifting schedules, particularly when the spring change removes an hour from the night.

2. What happens when a population lives permanently under one clock setting?
   This concerns the long-term effects of standard time compared with permanent summer time.

The evidence for short-term disruption around clock changes is stronger than the evidence directly comparing the long-term health effects of the two permanent options. Reviews of health outcomes have found evidence that is mixed and limited in places.

The American Academy of Sleep Medicine’s position statement supports permanent standard time, drawing on circadian biology and the available research. Still, that does not mean every person would sleep better under it, or that later evening daylight has no benefits.

The careful conclusion is:

Permanent standard time is the prevailing sleep-science recommendation—not a guarantee of better sleep for everyone.

And ending seasonal clock changes would not, by itself, resolve every timing problem. You could remove the twice-yearly disruption while still choosing a permanent clock setting that leaves winter mornings poorly aligned with daylight.

Spain’s extra question: is its standard time the right one?

For mainland Spain, choosing between its current winter and summer settings is only part of the debate.

Mainland Spain uses Central European Time in winter, one hour ahead of Portugal and the UK, even though much of Spain lies at similar longitudes.

Longitude describes how far east or west a place lies. It matters because the sun reaches different parts of the Earth at different times. A clock setting that fits a location farther east may be less closely aligned with daylight in a location farther west.

This is why Spain is sometimes described as being in the “wrong” time zone. But wrong is shorthand, not a literal scientific rule. Time zones are political and practical arrangements as well as geographical ones.

What actually happened in 1940?

Under Franco’s government, a March 1940 order advanced Spain’s clocks by one hour. That historical change is real.

The familiar claim that “Franco changed Spain’s time to match Hitler’s Germany,” however, states a motive more confidently than the order itself supports. The original order said the change would align Spain’s time with that of other European countries; it did not specifically name Germany.

The history helps explain how Spain arrived at its present clock setting. It does not, on its own, establish which setting would be best for sleep today.

Would moving to Portugal’s time help Spaniards sleep more?

Possibly—but not automatically. A time-zone change can alter the relationship between daylight and daily life, or it can amount to little more than relabelling the same routine.

To see the difference, imagine two scenarios.

Scenario 1: the clock changes, and schedules change with it

Suppose clocks move back one hour, but a workplace changes its start time from 09:00 on the old clock to 08:00 on the new one.

Those are the same actual moment. If school, meals and bedtimes also shift their clock labels in the same way, people would continue encountering daylight and sleeping at much the same actual times.

Changing the number on the clock does not create an extra hour of sleep.

Scenario 2: the clock changes, but the start time stays at 09:00

Now suppose clocks move back an hour and the workplace still starts at 09:00 by the new clock.

Work would begin one hour later relative to the sun. That could mean more daylight before work and less pressure to wake while it is still dark. Such a change could plausibly help align social schedules with the body clock.

But the result would also depend on what happened to finishing times, meals, evening activities and bedtimes. You cannot predict sleep duration from the time-zone label alone.

What does the Spain–Portugal comparison show?

A study comparing older adults in western Spain and Portugal found some more favourable measures of circadian regularity in the Portuguese group, but similar sleep durations.

That is a useful clue, not proof of what would happen if Spain changed time zones. Comparing two groups cannot isolate the effect of the clock from all the other differences in their lives.

Also, “Portugal’s time” needs clarification: Portugal and the UK currently change their clocks seasonally too. Moving to their winter time zone and abolishing seasonal changes would be two separate decisions.

The choice is about daylight and schedules together

If Spain ended clock changes, the clearest sleep-science recommendation would be permanent standard time rather than permanent summer time. The main reason is the value of morning light and the risk of pushing sleep timing later while wake-up obligations stay fixed.

There is also a plausible sleep-health case for bringing mainland Spain’s clock closer to solar time—the timing indicated by the sun. But that is a separate question from simply keeping its current winter setting.

Ultimately, the useful test is not just “What does the clock say?” It is:

• When does daylight arrive?
• When must you get up?
• When do school and work begin?
• Does your daily routine leave enough opportunity for sleep?

A better-aligned clock can help. Whether it produces more sleep depends on the schedules built around it.