Why does putting back the clocks an hour disrupt us so much?

The disruption of sleeping and waking patterns from the daylight saving clock change reveals a great deal about our everyday reliance on the interaction of sleep pressure and circadian clocks.

First, you need to understand the intricate changes happening in your body the night the clocks go back an hour. On Saturday evening, assuming we are not in bright light, our bodies will begin the daily chore of secreting melatonin, a key hormone for the timing of sleep. This will accumulate in the blood stream and a few hours later it will reach its peak concentration before declining steadily until morning.

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Melatonin does not make most of us sleep, and certainly doesn’t keep up asleep. It is more like a reminder, signalling that sleep should not be far away. Even brief periods of normal electric light delay or even stop this sleep signal, depending on its brightness and wavelength or colour.

In the evening as melatonin rises, the heat generated by our internal organs increases to its highest level of the day, followed by a drop – which is another sleep signal. This is why having a hot bath before bedtime can help us to sleep.

The body’s core temperature continues to drop for the first couple of hours of sleep, which is mostly slow wave sleep. This is when more of the neurons in the brain are firing simultaneously, and when our heartrate slows. It becomes more regular as we have this first episode of deep sleep. Our coldest core body temperature more or less coincides with the highest level of melatonin, showing the synchrony of these two circadian timing signals.

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A minute before 02:00 on Sunday 26 October our body’s timing systems and the
clocks will probably be aligned. Our internal core will be approaching its coldest temperature. As the body heats again, and the melatonin signal decreases, another circadian process begins – the slow sustained release of cortisol which will culminate on waking.

If melatonin is a sleep signal, then cortisol is a signal to wake. Unless we are very stressed during the daytime or drink a great deal of caffeine, it will be at its strongest at the time we typically wake. This is why waking up can sometimes seem both energising and stressful, and, why sleep is more difficult when we are stressed.

These three critical bodily timing systems, melatonin, core body temperature and cortisol, are synchronised by a central clock in the suprachiasmatic nucleus of the brain, which co-ordinates the time of the clocks in each cell of the body. The pattern of each signal repeats about every 24 hours, but can be disrupted by different aspects of our environment such as light, vigorous exercise and stress.

These cycles are not fixed at exactly 24 hours. They can be a few minutes shorter or longer than 24 hours. This enables our sleep-wake regimen to gradually change with the seasons.

But the change is slow. Abrupt changes, flying east or west (which extends or shortens sunlight exposure, affecting melatonin), heat waves, cold snaps (raising or lowering core body temperature) or stress (which increases daytime cortisol) cause disruption in this regimen. We just haven’t evolved to cope with sudden changes.

It will take days for the biological and actual clock to realign. Just as flying from London to New York takes more adjustment time than New York to London, the springtime change often feels gentler, because it seems to be easier to move your clock forward than backwards.

We are likely to lose out on sleep in the morning, particularly REM sleep, which kicks in later and is involved in emotion regulation. Our biological clock will still begin the cortisol-induced daily waking process at the same time it did the day before. But you will be awake as it peaks, which may resulted in deflated mood.

This disruption is not the same for all of us. About one in a 100 of the general population have a genetic disorder called delayed phase sleep syndrome, which makes it impossible to sleep until the early hours of the morning. Their melatonin levels increase much later than in other people, which means they will probably benefit from the clocks going back, if only for a short while.

Similarly, about ten to 20 in 100 late-adolescent children – compared to adults – are biologically driven to initiate sleep later. And for them, temporarily, their sleep may align more closely with the rest of the household. But they too will be sleepier in the morning.

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Another group in the population, about 1% of those in middle age, feel they need to go to bed far earlier than most, usually in the early evening, and wake very early in the morning. It isn’t clear why advanced-phase sleep syndrome is more frequent in this age group, although the circadian system seems to weaken as we age. This group is more compromised by clocks being put back.

The autumn clock change is also often difficult for menopausal women who experience hot flushes – their body clock appears to be advanced and tend to need to sleep earlier. Clocks going backwards mean they will need to wait longer for sleep than they might wish and wake earlier.

The daylight saving disruption rarely lasts more than a week. But one is left asking why we put our bodily clocks under this abrupt strain. We challenge the synchrony of our bodily clocks, for the sake of fleeting moments of additional light.