Sunlight and sleep cycles

The importance of the Sun didn’t disappear when we stopped using sundials. It’s extremely important to our health, both physical and mental. Chris Smith spoke to science journalist Linda Geddes, author of the book Chasing the Sun about the impact the Sun has on our health...

Linda - Well, sunlight affects us in various ways. There's Vitamin D as you mentioned, but there's also its effects on our circadian rhythms. So in every cell of our bodies, we have these (close to, but not exactly) 24 hour rhythms - in everything from when we release hormones, to the chemistry of our brains, to the activity of our immune cells. And the way those rhythms are kept synchronised with the time of day outside is through the action of light hitting this subset of cells at the back of the eye. But also the timing of those clocks is influenced by when we see the light. Most people will identify as either 'owls' or 'larks': people who like to stay up late, or people who like to get up with the sunrise. Now if you see lots of bright light early in the morning, your circadian rhythms are going to be shifted earlier, so you're going to become more lark-like; but if you're seeing light late at night in the evening and overnight, it's going to push you more towards being an owl. Genetics is also involved, so it's not just about light; but light does have this influence.

Chris - There must be an influence, then, of the invention of the light bulb! Because we haven't had artificial light until relatively recently in human existence, in the last hundred years or so. So has there been a big shift then?

Linda - Well, if you send groups of people camping, you notice that their circadian rhythms shift a little bit earlier, so they become more lark-like. But if you go and... when I was researching, chasing the sun, I went and visited an Amish family in America, and they have a much more traditional relationship with light because they're not connected to the electric grid, and they too are much more lark-like. And if you ask... there are some Amish people who will identify as night owls, but if you ask them, "what time would you ideally get up, and what time would you ideally go to bed?" I spoke to one of these people, she said, "ideally, I'd like to stay up until maybe about 10:00 PM? I know it's really, really late, but that would be my choosing." And she'd like to sleep in until 7:00 AM!

Chris - But why is it a problem, then, fighting your clock, becoming more lark-like, or becoming more owl-like? Why is that a problem?

Linda - It's not a problem if you can choose when you get up and go to work. So if you're not feeling sleepy until midnight, 1:00 AM, and you've still got to get up for work or school at 7:00 AM, you're cutting short your sleep. And sleep is important for all sorts of... for your health, for both your physical and mental health.

Chris - So it's the sleep that's paying the price, and the health cost comes because of sleep paying the price, is what you're saying - both in terms of the physiological cost, because we know that people who are sleep deprived have all kinds of physiological problems, high blood pressure, other risks of other things like weight gain and so on - but also mental health, because if you're not sleeping restoratively, you're more likely to suffer in that respect?

Linda - Yes. But there's a second thing, which is that if you're constantly shifting your sleep timing and you're not having that strong light signal in the daytime, and that weak light signal at night, your internal clocks can become desynchronised. So your clock in your heart isn't quite on time and in keeping with the clock in your stomach or the clock in your brain, and you're getting this kind of spreading desynchrony around the body. And that has been shown to have an influence on health independently of sleep as well.

Chris - When you were writing the book, did you also look not just at day-to-day time, but year-to-year time? Because there's also this phenomenon of Seasonal Affective Disorder. I suspect as the Northern Hemisphere goes into summer and we're actually seeing some sunshine, a lot of people will suddenly start to say they feel enormously better for no reason, but it's down to the longer days and sunnier days. Have you looked at Seasonal Affective Disorder as well?

Linda - Yes, I have. And there's an interesting thing about the increasing days though, which is that you assume that the most depressing time of the year is the winter, but when it comes to suicides and especially violent suicide, you get a peak in late spring. And the leading theory for why that is, is that you're getting this increase in serotonin, which is a brain signalling chemical which is produced in response to light as well. That's interesting. But yes, certainly Seasonal Affective Disorder is a real thing. And one of the best cures for it is light therapy, which involves being exposed to bright light, first thing in the morning, and what that is doing is resynchronising your clock to the 24-hour clock outdoors on Earth.

Chris - Have we seen consequences of people being cooped up during lockdown then? And is that part of the reason why people have found it tough - because they've been divorced from that normal, strong solar stimulus to feel good?

Linda - Well, I'm sure it's not the only factor, but one symptom of this kind of this desynchrony and this flattening of the circadian rhythm is that you feel sleepier in the daytime and more awake at night. Independent of the circadian clock actually, those light sensitive cells at the back of the eye also feed into brain areas that control alertness and mood as well. So light is a brain stimulant, but there've been studies that have looked at the effect of exposing people to an hour of relatively low intensity blue light, and being exposed to an hour of that is equivalent to drinking several cups of coffee in terms of the 'alertness boost' it gives you. What I always do is I try and get outdoors first thing in the morning and get some of that bright light first thing, but also try to get outside regularly during the day, just for a kind of light snack.