Jet Lag Planner

Morning Light or Evening Light: Which One Beats Your Jet Lag?

Picture two people taking the exact same bright morning walk on holiday. One shakes off the jet lag by the second day; the other makes it worse. The only thing separating them is the direction they flew in from. Whether you want morning light or evening light hangs on a single moment in your day: the coldest point of your night, which sits a few hours before you'd normally wake. Light in the four hours after that moment shifts your clock earlier, the fix for an eastward trip. Light in the four hours before it shifts your clock later, the fix for a westward one. Everything else is working out where that moment falls for you and making sure you don't get it backwards, because bright light on the wrong side of it will push your clock the wrong way and leave you feeling rougher.

The hinge is the coldest hour of your night

Your body runs a temperature rhythm around the clock, up in the late afternoon and sliding down through the night, and it bottoms out in the last hours of your sleep. That dip is the hinge everything swings on. Doctors call it your core-body-temperature minimum, or CBTmin, which is a mouthful, so let's stick with your low point. It's the coldest point of your daily temperature cycle, late in the night, and the direction light pushes your clock flips sides right around it. If you want the fuller rhythm it sits inside, Your Body Clock: Circadian Rhythm and the Temperature Minimum covers it.

The flip works like this. Light in the hours after your low point, from the dip on through your morning, tells your clock the day started early, so you get sleepy earlier that night and wake earlier the next day. That is an advance, in the jargon. Light in the hours before your low point, your evening and the first stretch of the night, tells your clock the day is running long, so you drift off later and wake later. That is a delay. The pull is strongest right next to your low point and fades the further off you drift, which is why light at midday barely nudges the clock either way. The real action is around your own dawn and dusk.

Flying east means chasing the morning and dimming the evening

East hands you a shorter day and an earlier schedule, so you need to shift earlier: get outside into bright light in the hours after your low point, which is your local morning once you have landed, and keep your evenings dim. East is the harder direction for most travellers, and there is a clean reason why. Your body clock naturally runs a touch longer than 24 hours, so left alone it would always rather stay up late than get up early. Why Flying East Is Harder Than Flying West gets into why that one quirk can cost you a week.

Flying west means soaking up the evening and hiding from early light

West stretches your day out and pushes everything later, so you want to delay. Catch bright light in the evening, before your low point, and shield your eyes from bright light in the very early morning, when it would yank you earlier and undo the progress you'd made. Delaying is the easy direction, so westbound jet lag usually clears faster and feels gentler than the trip out. If you have ever wondered why one leg of a round trip wrecks you and the other barely registers, that asymmetry is the answer.

The backfire nobody warns you about

This is the mistake that keeps people stuck for a full week. Bright light is the strongest lever you have for resetting your clock, which is the whole case made in Light: The Most Powerful Jet-Lag Lever, and that power cuts both ways. Fly east, then stride out for a bright dawn walk before your low point has passed, and that light delays you, the precise opposite of what you were after. Fly west, catch hard morning light too early, and you advance when you meant to delay. Point a strong signal at the wrong window and it just digs the hole deeper. So when you honestly can't tell whether you are in a chase window or a dodge window, treat it as dodge: sunglasses on, keep things dim. Shade never hurts, and that single habit will save you more than once when you've lost track of what day your body thinks it is.

Working out your own low point

You don't need a lab or a wearable for this. Take the time you normally wake and knock off about three hours, which is the rule of thumb the clinical protocols use; the CDC allows a wider two-to-four-hour window. If you are usually up at seven, your low point sits around 4am, give or take an hour; wake at six and it is nearer 3. You don't need to nail the exact minute either, because the seek and avoid windows run four hours either side, so getting the direction right, before your low point versus after, matters far more than pinpointing it to the quarter-hour.

Two things do catch people out, though. The first is that your low point won't stay put: every day you successfully shift, it moves with you, roughly an hour a day in whatever direction you are heading, so work from the sleep you're actually getting rather than the clock on the destination wall. The second is that larks and owls start from different places, so an early riser has an earlier low point and a night owl a later one, and you should nudge your estimate toward your own real sleep pattern rather than some textbook average. Larks and Owls: How Your Chronotype Changes the Plan helps you place yourself.

Fun fact

Your core body temperature swings by only about half a degree Celsius across an entire day, a wobble so slight you would never feel it happen. Yet the very bottom of that tiny dip is the switch that decides whether a morning of bright light heals your jet lag or deepens it. Half a degree, deciding the whole thing.

How the Jet Lag Planner handles this

Estimating your low point by hand and then tracking how it creeps across a shifting local clock, day after day, is exactly the fiddly arithmetic nobody wants to be doing on holiday, so the Jet Lag Planner does it for you: from your usual sleep and your flight details it pins down your starting low point, projects where it moves each day, and prints plain chase-light and dodge-light windows in local time. It is a planning aid, not a diagnosis.

Sources

General wellness information, not medical advice. Talk to a clinician before changing your schedule if you have a sleep or health condition.

Last reviewed August 5, 2026.

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General wellness information, not medical advice. Talk to a clinician before changing your schedule if you have a sleep or health condition. © 2026 XOR Limited.