Tuesday, 16 December 2014

"Light Lag" - Seasonal Jet Lag From Flying North/South

I am a medical science pioneer, because I think I've found a black hole in medical knowledge: jet lag from flying north or south.

This week I flew my 10th mega-haul flight, i.e. 24+hrs and 2+ plane trips.

This one was 25hrs from Melbourne to London. 25hrs is about the shortest flight you can do between the UK and Aus/NZ, and this felt like one of the easiest.

So why do I still feel terrible 5 whole days later?

Because I've gone directly from warm long bright days to freezing short dark days.


Everyone says "West is best" for jet lag. This is science, because the body (apparently) adjusts to longer days more easily than shorter ones.

I've always been confused by this, because I've found it easier flying East to New Zealand than flying West to UK.

Sure, NZ is not technically "East" - it's on the opposite side of the world, +12hrs from UK.*

But if it's the same time change going each way, why have I always felt great getting in to New Zealand, but awful adjusting back in the UK?

6 of these mega-haul flights were in UK autumn and NZ spring - between September and November.

  • Each time flying NZ to the UK, the daylight and temperatures were about the same. Cue standard jet lag for 3-4 days, including tight muscles, feeling generally rubbish and low energy.
  • Each time flying UK to NZ, the daylight was much increased - leaving darker, colder days for longer, warmer days. And I felt great - slightly upset sleep patterns for 2-3 days, but energised and upbeat during the daytime.

The other 2 flights were in July/August, which is NZ winter and UK summer. I definitely felt the effects going both ways, but I don't remember much difference between them.

Are there any studies? Does anyone care?

Okay, this is all anecdotal, and as a rationalist I often remind people that anecdotal evidence is not evidence.

But a quick look around the internet suggests no-one seems to be taking this idea seriously - that seasons affect jet lag.

Wikipedia hilariously says...
"North–south flights that do not cross time zones do not cause jet lag"
...while also saying...
"Light is the strongest stimulus for re-aligning a person's sleep-wake schedule"
Which is funny, right? Because when it says...
"A ten-hour flight from Europe to southern Africa does not cause jet lag, as travel is primarily north–south"
...you'd think light could be a major factor if you're flying in June, from the longest days of the year in Europe, to the darkest days of the year in southern Africa.

The same goes for Sydney and Tokyo, Santiago and New York, and so on.

Do we need another name like "Light Lag"?

If you're in the same time zone, midnight is still midnight and noon is still noon.

And if you're flying 2 hours north/south, near the equator, even at midsummer or midwinter the difference won't be very strong. You might not even notice it.

But if your departure and destination are both quite far north and south on the planet, the time that the sun rises is going to be pretty different, especially in June or December. And your body will notice it.

  • If you fly Cape Town to Helsinki in December, your time zone is the same, but the sun rises 4 whole hours later.
  • So in terms of light, how is this different from flying 4 time zones West?
  • Added to that, the day isn't just shifted forward a few hours - it ends 5 hours earlier too. So your body has 2 different things to work out (as well dealing with a massive drop in temperature!).

With jet lag, your body only has to move its cycle by a few hours. If you're staying on the same longitude, the day pattern is the same, and your body can adjust easily.

In contrast, rapid seasonal change means your body has to change its entire response cycle, because times between light-dark and dark-light are both completely different.

To me, it seems clear that "jet lag" should either include this seasonal effect of north/south flying - which current discussion clearly ignores - or we need another term like "light lag" to account for it.

So if there isn't already a term, I'm claiming one. I officially declare I have coined a new phrase "light lag". Consider this as me sticking a little linguistic flag in it. (Or RSC "Rapid Seasonal Change". That sounds a bit more legit.)

Boom, I just medical-scienced all over this blog.

Emotions and warmth

In fairness, your emotional state can also affect your body's health and performance.

So, flying to New Zealand always gives me feelings of a) excitement, from the idea of starting a new adventure, and now also b) happiness, from the sense of "coming home" to a country I have very happy memories of.

Then again, I always feel happy coming back "home" to England too, to see my UK family and friends.

More definite is that landing in a warm climate or season (e.g. Thailand or NZ spring/summer) helps your body get around and do things. Right now, my body is tight and tense, and I think that's directly related to going from warm Australian spring to the 0-5 degrees of UK December.

This is my first UK December since 2007 and I am cold!

No-one cares about the Southern Hemisphere...

Just finally, one of my personal bugbears from living 6yrs down under is how northern-hemisphere-oriented the world is.

I think Kiwis and Aussies are used to it. They grow up with a British colonial cultural heritage, they get up in the night to watch sports matches. They're perfectly used to the news talking about things happening in a different hemisphere.

Me on the other hand, I'm a demanding European. Everything usually happens in my hemisphere!

So when it's solstice in June, the whole internet is talking about "summer solstice" - completely forgetting it's winter for literally half of the planet.

And there's the mental block Americans and Europeans have about millions of people celebrating Christmas in summer. Everyone in Europe and the USA knows this, but they find it very hard to focus on, like something that's always in the corner of your eye.

  • Unless the novelty of beaches and barbecues is specifically the topic of conversation, Christmas is always in winter - that's just how it is.
  • Likewise, unless the southern hemisphere is specifically the topic, the default hemisphere is the northern hemisphere.

To be fair, the southern hemisphere only has 12% of the world's population. That's Australasia, Oceania, South America and southern Africa. The northern hemisphere has most of the land and most of the human race: northern Africa, Europe, almost all of Asia, and North and Central America.

So maybe it's no wonder that there's very little attention to north-south flights across significant changes of latitude.

But hey, don't forget us down past the equator.

* It's usually +11hrs (UK summer) or +13hrs (NZ summer) because of daylight savings in both countries.


  1. Hi yeah get it all the time, I fly to Japan, I find northern winter southern summer the worst, I don't fall asleep at strange times but I am lethargic and dizzy

  2. Great blog post and really helpful...... and your blog are very interesting and inspiring.


  3. I’ve just found this blog post because I was googling the effects of flying from the
    Longest day to the shortest day. I just left the UK six days ago, flying London to Melbourne, just one week after midsummers night - in the UK we were having great weather and daylight was 18 hours of sunshine, followed by 6 hours of pale light in the night sky. We arrived back in Melbourne to cold weather and total darkness lasting 14 hours, with 10 hours of watery winter light and a few hours of sunshine. I have never felt this bad with jet lag - queezy, very tired and my sleep is all over the place. I usually recover well, within three days. This is day six and I feel awful. You’re onto something!