Daylight Saving Time: Why the Clock Change Hits You

On the Sunday the clocks went forward, Daniel woke up and his phone insisted it was an hour later than he had expected. Somebody had taken an hour of his sleep overnight without asking, and he spent the rest of the week paying it back with interest. On Sunday evening he could not fall asleep, on Monday the alarm went off in the middle of his deepest sleep, and by Thursday he still had the feeling that his whole week had been set down one step to the side.

A time change looks like an administrative detail. For the body it is a small time zone you flew into without leaving the house.

Why one hour behaves like a long-haul flight

The switch moves exactly one thing: the numbers on the clock. The internal clock in your brain does not budge by a single minute. What opens up is a gap between social time, the hour school or your shift starts, and biological time, which runs sleepiness, hunger, body temperature and attention.

Your internal clock is set mainly by light. Morning light pushes it earlier, evening light holds it back. Meal times and physical activity play a supporting part. So does whether someone is talking at you at ten at night. Melatonin, the hormone the body starts releasing once it gets dark, does not appear any earlier just because a dial was rewritten on Sunday night.

A rough rule from travel jetlag research says roughly one day of adjustment per hour of shift. In spring it usually takes longer than that. The German chronobiologist Till Roenneberg and his colleagues described in 2007 that internal clocks essentially fail to reset to the spring shift, and that for a portion of people the mismatch between biological and social time drags on for weeks. Long spring evenings are full of light that holds the clock back, while the alarm keeps pushing it forward.

The result is the state people describe as "no energy for anything" or "not quite here". It is not about one bad night. It is several days of sleeping at a time your body is not tuned to.

Spring hurts more than autumn

Most people describe the autumn shift back as pleasant. An extra hour, a longer Sunday morning, and once a year the sleep debt gets partly wiped. The spring shift is the opposite trade: you lose an hour of sleep and on top of that you have to get up at a time your body still counts as night.

Here comes the part that surprises almost everyone. Human internal clocks do not tick at exactly 24 hours. Charles Czeisler's team at Harvard measured in 1999, under conditions with no cues about the time of day, that the average internal period runs slightly over 24 hours. Our clocks have a built-in tendency to drift later, which is why going to bed later comes more naturally to us than going to bed earlier. The spring shift runs straight against that tendency. The autumn one runs with it.

That this shows up in people is not just the impression of tired employees. The Canadian psychologist Stanley Coren published an analysis of traffic accidents in 1996 in which he found an increase of a few percent on the Monday after the spring shift compared with ordinary Mondays. After the autumn change the effect did not appear. Research also suggests a slight rise in some cardiac events in the first days after the spring shift, though the numbers differ between studies and are worth treating carefully.

Put together: the spring Sunday is not the same animal as the autumn one, even though both go by the same name.

Who the switch throws off most

The answer sits in where a person's internal clock is set. Owls, people with a naturally late rhythm, struggle to fall asleep "on time" without any shift at all. The spring change makes that problem sixty minutes bigger. In the morning they then get up at an hour their body reads as the middle of the night.

Larks usually walk through the spring shift. Autumn is what bothers them: the light disappears early in the afternoon, the evening wind-down arrives sooner, and by half past eight they are done for the day even though the social programme is only starting. Doves, who are the largest group, typically take a few off days and then settle.

The band is not the whole story. Alongside the morning-or-evening question there is a second axis: how steady or flexible your rhythm is. Some people retune almost overnight and survive a Sunday flight home from another time zone with no aftermath. Others have their clock nailed down and are thrown off even by getting up an hour later on one Saturday. It is precisely people with a steady rhythm who report the longest tail after a time change, even when they sleep well otherwise and sit in the middle of the lark-owl axis.

The shift lands especially hard on small children. There is no explaining to a two-year-old that today they should go down an hour earlier because a calendar said so. Parents then handle several evenings where the child is not sleepy and several mornings where they will not get up. Shift workers are in a similar position, with a rhythm that is already unsettled: the switch adds one more mismatch to the one they are wrestling with anyway.

There is a fairly precise term for this state. The gap between when a person would sleep by their own body and when they have to sleep is described by social jetlag. A time change is really its society-wide edition: twice a year everyone gets the same dose, and everyone handles it differently.

What you can do a few days ahead

The simplest way to make the week after the switch easier is to refuse to take the whole hour at once. Split it into smaller pieces and start three or four days early.

• Move your bedtime and your wake time by 15 to 20 minutes a day, in the direction the clocks are going.
• Move meals along with sleep. Dinner and breakfast are a weaker signal for the internal clock than light, but they are not nothing.
• On the morning of the change, get outside, ideally within 30 to 60 minutes of waking. Outdoor light is incomparably stronger than indoor lighting even under cloud.
• In the evening, dim things down instead. Less overhead light, fewer bright screens near your face, no work calls still running after ten.
• Skip coffee after three in the afternoon. In a week when sleep is already crooked, an afternoon coffee shows up in how you fall asleep more than it usually would.
• In autumn do the mirror image: keep morning light lower and make use of evening light so it does not put you to sleep at seven.

With children the same applies, only slower and with more patience. Moving bedtime in ten-minute steps four days ahead works better than one big jump on Sunday, because a child has no way of knowing why they should be sleepy earlier. It helps to keep the rest of the ritual unchanged, the same story and the same order of steps. In the morning, open the curtains as soon as you can and let light in, ideally instead of a lit screen at breakfast.

Anyone who does not manage to plan ahead has lost nothing. Even the Sunday on its own does a good chunk of the work if you get outside in the morning and skip anything in the evening that revs you up.

The first days after the change

The Monday after the spring shift is the worst day of the entire year for demanding decisions. Reaction times are slower and attention slips sooner. Tiredness also arrives at a different hour than you are used to. This is not about your discipline. It is that your brain is running on an internal time that lags behind the one on the display.

An honest question: are you scheduling an important meeting, a presentation or a long drive for exactly that Monday after the change? Most people have never weighed it up, because a shift in the calendar looks harmless. Yet moving the same item two days later means working with a considerably better version of yourself.

The trap of the first week is called the weekend. After five days of short sleep, Saturday arrives, you take a longer morning, and by Sunday evening you are back at the start. Do allow yourself a bit of extra sleep, but half an hour rather than three hours. A short top-up evens out the gap, a long one deepens it, because you push your clock later again.

A practical minimum for the first week looks like this. Go to bed by the clock rather than by feel, because feel will send you to bed late all week. Cap an afternoon nap at twenty minutes, otherwise it eats your evening sleepiness. And behind the wheel it pays to assume that you and the drivers around you are a little slower than you think.

Scrap the switching, or keep it

The debate about ending the clock changes has been running for years, and the political part belongs in a different article. Europe moves its clocks on the last Sunday in March and the last Sunday in October, North America on the second Sunday in March and the first Sunday in November, and both have spent years arguing about whether to stop. More interesting is that among circadian rhythm researchers there is fairly broad agreement on which option would be better if the switching went away: permanent standard time, the winter one. The reason is simple. Standard time keeps the midday sun closest to actual noon, and people get more light in the morning, when it is most effective for setting the clock. Permanent summer time would mean dark winter mornings for months on end, which schoolchildren and people on early shifts would pay for most.

How much the switch throws you, though, does not depend only on what the authorities decide. Where your internal clock sits and how stable your rhythm is both matter. Some people retune almost overnight, others are unsettled by an hour of drift over a weekend. If you have no idea where you belong on that axis, a few minutes with our chronotype test will give you a rough picture. What a chronotype actually is and how it changes over a lifetime is covered in more depth in the article on the chronotype.

There is another angle worth noticing. A time zone is the same across its whole width, but the sun does not rise at the same moment at both edges of it. The difference can run to tens of minutes, so an identically set alarm goes off at a different phase of the morning in a city on the eastern edge than in one near the western boundary. The time change only sharpens that inequality.

How it turned out for Daniel

The following spring Daniel decided to try one thing. From the Wednesday before the change he went to bed twenty minutes earlier each night, and on Sunday morning he drank his coffee on the balcony instead of at the laptop. On Monday he was not fresh. But for the first time in years he did not need two extra afternoon coffees to get to Friday.

The cheapest intervention in the whole week costs ten minutes of morning light. Your internal clock goes by what it sees shortly after you wake, not by what you tell it about what time it is.