If you’re thinking this is a blog about the light emitted from screens, you’ve knocked on the wrong door. You’re looking for this 4-part series.

This is a blog about the light that us human moths don’t look directly at.

That’s the evening light that comes from living in the city, from those overhead lights in our house, and those long summer days …

Light, Latitude and Sleep

The idea that we prepare for sleep when the sun sets doesn’t work for some humans walking this place called Earth.

For example, for those wonderful people who harvest the land of wonderful products that get turned into beautiful chocolate and coffee (ie, Ecuador and Columbia) - they live right near the Equator. This means they reliably receive 12 hours of light all year round.

The further you live away from the Equator, the more variable the length of your day is.

In one study we performed, we compared the relationship between the length of daylight and sleep, by comparing young people living at different latitudes.

For example, we collected data from those living 36 degrees away from the Equator (Adelaide, Australia), 52 degrees away from the Equator (Amsterdam, The Netherlands), and 69 degrees away (Tromsø, Norway)

To give you some perspective of where the city of Tromsø lies, it’s about 300 km within the Arctic Circle. It’s so far away that the Avengers safely hid Natalie Portman’s character there in The Avengers movie (clever title hey?).

So Tromsø, is way out of the inter-planetary-travelling Thanos’ prying eyes - yet still comfortable enough so that Natalie could dine at the local Burger King (although I hope she has patience because - for a fast food joint - they take a long time to make food. I know because I’ve been there) …

Anyway, we found that ‘daylength’ provided a small contribution to young people’s sleep (Bartel et al., 2016).

To provide some perspective, young people’s sleep is affected more by alcohol and tobacco (Bartel et al., 2017) - regardless of what latitude they drink or smoke.

Seasons and Sleep

Instead of comparing groups of people across different latitudes, another way of seeing whether Outdoor Light affects sleep is to track the same group of people across a year, and see if the seasonal fluctuations in light affect their sleep.

The advantage of this approach is that when you find differences between people across latitudes, you don’t really know if the differences are due to Outdoor Light or some other cultural difference.

One of the best places on Earth to track the same group of people across seasons are those who reside really far away from the Equator.

You guessed it - like Natalie Portman living in Tromsø, Norway!

Well, because of this thing called ‘research ethics’ I cannot divulge whether Natalie Portman was in our Tromsø study or not.

But what I can say is that in Tromsø, people don’t see the sun for a couple of months over winter, yet the sun doesn’t set in their summer.

But these extreme seasonal fluctuations don’t have a huge impact on sleep (Friborg et a., 2014).

Light and Niteowls

The problem with generally looking at seasons or daylength is that the environmental light can be a mix of natural light (the sun) and artificial light (not the sun).

Thomas Edison has been considered by some to be the serial killer of modern sleep. I mean, have a look at the guy, and tell me he doesn’t look like a serial killer of some persuasion …

Edison invented electricity - and it has been proposed that living in areas dense with electric lighting causes sleep problems.

I remember a classic study examining the sleep of young people who went to school in Sao Paulo, Brazil - yet some went to their 24/7 electrified homes, and others went back to their home where the generator was turned off at night (Peixoto et al., 2009).

The teens that lived in rural areas without 24/7 electricity slept more (and earlier) compared to their electrified peers.

But one study does not maketh a fact.

Whereas another study found that teenagers living in the city were more nite-owl-ish compared to their peers in rural areas (Vollmer et al., 2012), no connection between outdoor light-levels and nite-owl-ish-ism has been found (Hurley et al., 2013).

And as yet, there has been no meta-analysis (ie, a study that analyses the data of all studies) quantifying whether environmental light at night affects humans’ sleep.

Conclusions?

Light at night can have serious effects on human health. Although the focus of this blog has been on sleep, there are meta-analyses that have shown a link between LAN and breast cancer.

Specifically, there is a 12-13% increased risk of developing breast cancer with increased exposure to outdoor or indoor light at night (Lai et al., 2021).

So despite the fact that I’m not completely convinced of a consistent link between environmental light and sleep, this doesn’t mean you should take that nightshift job at your local gas station.

If there are inconsistent and small effects, it suggests that people’s sleep respond differently to LAN.

But quite frankly, who wants to be under bright lights at night?

Even those who are starved of light during the day (ie, those living in the far north or south of our planet), still find the balance between light and darkness across the 24-hr day.

So this week, maybe think about how you can find a nice balance between light and dark in your night-to-night lifestyle?

Enjoy a candlelit night - just snuff out the candle(s) before you go to sleep!

Maybe you’re one of those who benefits from being LAN-less …

• Prof MG