Why does jetlag affect memory?
This question is courtesy of Cat, who asked me about memory and jetlag after a long-haul flight from the UK to Australia. I was tempted to claim that all airports are cursed (which is clearly true; just look at any duty-free section) but then I started reading and it turns out there is a rational explanation, though I suppose that doesn’t preclude the possibility of a rational-explanation-plus-curse double whammy.
And so, let us head even unto several psychology journals and seek therein our answers.
Freshly-cursed luggage, just retrieved from a baggage carousel in the shape of the sigil that summons the ichor god Bel-Shamharoth
You have probably noticed that humans as a species like to be awake when it’s light and asleep when it’s dark. This is called diurnality, and it is seen in lots of other animals like butterflies and squirrels. Other types of sleep cycle are possible, including crepuscular (awake at dawn and dusk, like cats), matutinal (awake early in the morning, like some bees), and nocturnal (awake at night, like your neighbour’s baby). Sleep cycles are often a matter of circadian rhythms, which are internal bodily processes like the release of hormones, but these circadian rhythms in sync with external factors called zeitgebers, like how light it is, when you last exercised, and whether you’ve recently interacted with others.
So: you have put yourself on an aeroplane from the UK to Australia. You are about to cross many time zones in artificial light and dark, during which time you will largely remain seated, while your boyfriend distracts you from how afraid you are of flying by talking to you constantly about which dog is the best dog (… that’s just me, isn’t it?). Your circadian rhythms are about to lose all track of their zeitgebers. No wonder jetlag feels so awful.
There is a group of people who put themselves through jetlag regularly, and you will find them working on your aeroplane. No, not self-important businessmen: the pilots and the cabin crew. When Cho, Ennaceur, Cole and Suh [i] wanted to know the effects of jetlag, it was the cabin crew they turned to – I assume because there are often fewer pilots than cabin crew on any given aeroplane.
Cho and colleagues asked the women who took part in this study to take saliva swabs once an hour during their working days for an entire two months, which frankly would be a tax on my memory even without jetlag. The reason they did this was to keep track of cortisol, a hormone that is released in response to stress. It will probably not surprise you to know that cortisol levels (and therefore stress) were higher in air crew (who crossed time zones) than ground crew (who stayed in one place), but it’s not clear whether this happened because of jetlag in itself, or because people were anticipating the horrible experience of jetlag, or even for some other reason like ‘being in a big metal tube that’s inexplicably airborne is inherently stressful’.
What was clear was that the air crew didn’t become less stressed over time: someone who’d been doing the job for four years had just as much cortisol as someone who’d been doing the job for just one. Furthermore, the people who’d been doing the job for four years were starting to have memory problems, probably as a consequence of having elevated cortisol levels almost all the time – but this wasn’t the case for employees of every airline, which might be because different airlines have different minimum periods between flights for their crews. We might also want to consider that memory gets better and worse over the course of the day in many different species, perhaps because of hormonal cycles [ii], so messing with these hormonal cycles by, say, getting on a long-distance flight might well influence memory regardless of stress.
Cho and colleagues’ study was rather unusual because all of their participants were women. A lot of studies on circadian rhythms have concentrated on men, because women tend to have pesky womb-related hormonal cycles and apparently no-one has yet said, “They’re a feature, not a bug!” and “Men have hormonal cycles as well!” and “Have you considered that trans people exist?” enough times to have any effect on the situation. Luckily, Santhi and colleagues [iii] did their best to combat this bias, so we do at least know that (cis) men and (cis) women are largely similar in their circadian rhythms, but that (cis) women tend to have greater variation than (cis) men in how accurate they are at tasks over the course of the day – which probably also means that they are affected more by jetlag.
It’s not just jetlag that can cause problems for circadian rhythms. When doctors have to work 80 or more hours a week, they tend to unintentionally drift into sleep much more often than when they do not have to work 80 or more hours a week [iv]. I’m sure we can all agree that it is a good idea for doctors to not literally fall asleep while treating people, and indeed in the EU the Working Time Directive now prevents employers from working doctors that hard. (Apparently Britain is keeping this after we leave the EU, assuming the NHS doesn’t implode under the pressure.)
“I’m Dr Peter Velcro, and I’ve not fallen asleep at work for over 20 years! Now you too can experience the miracle of worktime wakefulness with this one easy trick.”
Similarly, people working night shifts in a factory are much more likely to have accidents than people working day shifts [v], but unsurprisingly there is much less legislation to protect night shift workers than there is to protect doctors [insert four-hour socialist rant here].
What can we do to make adjusting to jetlag or shift work easier? To some extent, this is a matter of genetics: some people just adapt more quickly than others. However, Arendt [vi] suggests a few things that can help someone facing a multi-time zone flight. If you’re only staying a couple of days in your destination, don’t bother trying to adapt to your new time zone if you don’t have to – instead, just use caffeine, naps, and all the other strategies you used to get yourself through exams. For trips of a week or so (and night shift work, which tends to be on a weekly cycle), adjusting towards your new schedule but not all the way to it might help; for example, if you’re doing an 11pm-5am shift, you might go to sleep around 3pm, so that at least some of your sleep happens in the dark. Lastly, for long trips where you do want to adapt fully, you can try to position yourself in the aeroplane so you get light from the window if you need to stay up later (or vice versa to go to sleep earlier), or take the hormone melatonin to help regulate your sleep and wake cycle.
Elbow the person next to you until they cede the window; it’s the only way
Want to avoid jetlag entirely? Arendt suggests that you gradually adapt to your new time zone before you go on your flight, sleeping an hour earlier or later every night until you’re in your new time zone. Of course, not everyone has the luxury of being able to do this, so I have some much simpler advice: just don’t get on the plane in the first place.
[iii] Santhi, N., Lazar, A. S., McCabe, P. J., Lo, J. C., Groeger, J. A., & Dijk, D. J. (2016). Sex differences in the circadian regulation of sleep and waking cognition in humans. Proceedings of the National Academy of Sciences, 113(19), E2730-E2739.
[iv] Lockley, S. W., Cronin, J. W., Evans, E. E., Cade, B. E., Lee, C. J., Landrigan, C. P., ... & Aeschbach, D. (2004). Effect of reducing interns' weekly work hours on sleep and attentional failures. New England Journal of Medicine, 351(18), 1829-1837.