How Time Changes Affect Your Brain and Your Work

Quick answer: Crossing time zones, switching shifts or changing the clocks can leave you trying to sleep while your brain is promoting wakefulness, then working through your biological night. Lost sleep adds to the mismatch, affecting attention and emotional regulation, so plan the adjustment and the work you’ll do during it.

Fly somewhere five hours ahead of home and a 9am presentation on your first morning takes place at 4am by your body clock. Everyone else has started their day, while your brain still expects to be asleep.

We plan work in local time because it gives everyone a shared schedule. But the same hour asks very different things of someone who’s just stepped off a long-haul flight, or someone who’s lost sleep to early calls with another time zone, and with the UK clocks going back on Sunday 25 October, most of us are about to get a small taste of it.

Being available for a meeting and being in a good state to contribute to it are two different things, and knowledge work depends on the second. Here’s how shifts in timings impact our brains and what that means for work.

Your body keeps its own time

Circadian rhythms are the roughly 24-hour cycles that shape your sleep, hormone release, digestion and body temperature. They’re coordinated by a small region of the hypothalamus called the suprachiasmatic nucleus, which takes in information about light through your eyes and keeps clocks throughout the body in a regular rhythm (NIGMS, 2025).

Melatonin normally rises in your biological evening as a signal that night is coming, while a circadian drive for wakefulness keeps you alert through the day. Alongside that, sleep pressure builds the longer you’re awake and is released while you sleep (NHLBI, 2022a). On an ordinary day, the two keep you sleepy at bedtime and alert in the morning.

After a time change, these natural rhythms pull against your new schedule. You can be exhausted after a long journey and still lie awake at the local bedtime because your body clock is promoting wakefulness, then have your alarm go off while your body still expects to be asleep. Even your appetite can feel out of step, which is why jet lag can bring digestive discomfort too.

So two problems are tangled together: circadian misalignment, where your internal timing and your schedule disagree, and sleep loss, where that disagreement or the journey itself cuts your sleep short. They’re connected but distinct, which is why a long lie-in can repay some lost sleep without moving your body clock any closer to local time.

Not every time change asks the same of you

Crossing several time zones puts your body clock out of step with local light and dark, and jet lag is the temporary disruption that can follow.

Direction matters too. Flying east usually means bringing your sleep earlier and flying west means moving it later, and because most people find it easier to delay than to advance, westward trips tend to be easier to recover from. Distance, light and individual differences play a part too, but direction is worth factoring in when you schedule demanding work after landing (Riedy and Williams, 2025).

Shift work creates the same problem without any travel: moving from day shifts to nights creates a substantial mismatch, and an earlier start can cut your sleep short if your bedtime stays put.

Seasonal clock changes are the smallest shift of all. In March the UK moves from Greenwich Mean Time (GMT) to British Summer Time and the clocks go forward, so we lose an hour in bed. On the last Sunday in October the clocks go back to GMT, which gives us an extra hour in bed, though not necessarily an extra hour of sleep. The British Sleep Society would like the UK to stay on GMT all year and stop putting the clocks forward each spring (Crawford et al., 2025). Their reasoning is that morning light is what keeps our body clocks in step with the day, and summer time means getting up an hour earlier relative to sunrise, so we see less of it. If you track your cycle, our guide to protecting your cycle through the clock change explains how this month’s change can affect it.

What happens in your brain

How alert you are depends on two things at once: where you are in your body clock’s daily cycle and how long you’ve been awake. A brain-imaging study shows both at work: researchers scanned the brains of 33 people doing attention tasks across 42 hours without sleep. Activity in many brain regions rose and fell over the day in time with the body clock. On top of that daily rhythm, activity in much of the cortex, the outer layer of the brain where most of our thinking happens, kept falling the longer people stayed awake (Muto et al., 2016).

In a separate laboratory study, researchers shifted people’s sleep-wake schedules by 12 hours. Their sustained attention got worse, yet their ratings of their own performance didn’t reliably reflect the drop (Chellappa et al., 2018), so feeling capable after a time change isn’t always a fair guide to how well you’re working.

Work also means handling other people. In a small experiment, people kept awake for around 35 hours showed stronger responses to negative images in the amygdala, the part of the brain that helps flag emotional significance, and that activity was less closely coordinated with the medial prefrontal cortex, which helps regulate emotional reactions (Yoo et al., 2007). In other words, losing sleep can make you more sensitive when things go wrong and less able to keep your reaction in proportion.

These experiments involved far bigger disruptions than a one-hour clock change, but they do suggest that a difficult conversation the morning after an overnight flight may ask more of you than its slot in the diary implies.

Plan the work around the adjustment

Start with the work that would be costly to get wrong. A negotiation straight after landing, or a final check of the numbers after several broken nights, deserves more thought than whether there’s a free slot. Where you can, build in recovery time before the main event and keep big decisions away from the hours when you’d normally be asleep.

If the schedule can’t move, change how you approach the task. Write your decision criteria down in advance and ask someone to check the important details.

Managers can help by rotating awkward call times across international teams and protecting recovery time after work trips. Holding on to a sensible bedtime is much easier when the whole team takes biology seriously.

Help your body adjust

Before a big time-zone change, shifting your sleep gradually towards your destination’s time can help, though for very short trips fully adapting may not be worth it because you’ll be home before you’ve finished (Riedy and Williams, 2025).

Light is the strongest signal your body clock responds to, and the timing decides which way it moves. Bright light in your body’s morning pulls the clock earlier, while bright light in your body’s evening and the first half of its night pushes it later. After a big jump east, those windows fall at unexpected local times. Fly eight hours east and 8am local time is midnight for your body, so bright light then can push your clock later, the opposite of what you need. A light plan built for your route, telling you when to seek light and when to avoid it, is more useful than a blanket rule to get outside as soon as you land (Eastman and Burgess, 2009).

Whatever the change, give yourself enough time to sleep and don’t make up for a slower day by working later. Keep caffeine well away from your planned bedtime and any nap short enough not to spoil the next night. For the seasonal clock change, a reasonably regular routine is the best place to start (NHLBI, 2022b).

How to use this with Phase

Phase reads your bio data, including your sleep, HRV and cycle data, translates it into a cognitive profile and maps that against your tasks and calendar to produce an alignment score.

Around a time change, look at how long and when you’ve slept across several nights, alongside how you feel. Our guide to what your readiness score means for work explains why the inputs matter more than the headline number. Your wearable’s sleep estimate doesn’t measure your body clock directly, though, and an alignment score can’t tell you your jet lag has gone.

Use what you see to make one specific choice, such as moving a final decision or asking for a second pair of eyes. There’s more on the science behind Phase, and more guides on wearables, sleep and energy.

Leave room to adjust

Whenever your schedule shifts, give sleep and adjustment a place in the calendar before you fill every free hour. Join the waitlist and be first in when Phase opens up.

References

National Institute of General Medical Sciences (2025) Circadian rhythms. Available at: nigms.nih.gov.

National Heart, Lung, and Blood Institute (2022a) How sleep works: your sleep/wake cycle. Available at: nhlbi.nih.gov.

Riedy, S.M. and Williams, S.G. (2025) 'Jet lag disorder', CDC Yellow Book 2026. Available at: cdc.gov.

Crawford, M.R. et al. (2025) 'The British Sleep Society position statement on Daylight Saving Time in the UK', Journal of Sleep Research, 34(3), e14352. doi: 10.1111/jsr.14352.

Muto, V. et al. (2016) 'Local modulation of human brain responses by circadian rhythmicity and sleep debt', Science, 353(6300), pp. 687-690. doi: 10.1126/science.aad2993.

Chellappa, S.L., Morris, C.J. and Scheer, F.A.J.L. (2018) 'Daily circadian misalignment impairs human cognitive performance task-dependently', Scientific Reports, 8, 3041. doi: 10.1038/s41598-018-20707-4.

Yoo, S.S., Gujar, N., Hu, P., Jolesz, F.A. and Walker, M.P. (2007) 'The human emotional brain without sleep: a prefrontal amygdala disconnect', Current Biology, 17(20), pp. R877-R878. doi: 10.1016/j.cub.2007.08.007.

Eastman, C.I. and Burgess, H.J. (2009) 'How to travel the world without jet lag', Sleep Medicine Clinics, 4(2), pp. 241-255. doi: 10.1016/j.jsmc.2009.02.006.

National Heart, Lung, and Blood Institute (2022b) Sleep deprivation and deficiency: healthy sleep habits. Available at: nhlbi.nih.gov.

Medical Disclaimer

The information in this post is for educational purposes only and does not constitute medical advice. If you have concerns about your health or wellbeing, please consult a qualified healthcare professional.

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