The power of light to change your world for good
Light Notes banner

Seasonal shifts, efficiency v. effectiveness – and a book!

Solar or social time?

As we start to feel the winter chill and it gets dark so early here in the Northern Hemisphere that most of us go to work before the sun comes up and it’s dark before we log off for the day. So shouldn’t we all just curl up and go to sleep?

In civilisations without electric light, the body clock does change with the seasons, as this study of seasonal changes in hunter-gatherer populations Tanzania, Bolivia and Namibia shows. Natural Sleep and Its Seasonal Variations in Three Pre-industrial Societies. But for the rest of us, our body clocks stay locked to social time, driven by exposure to artificial light and social patterns – Weekly and seasonal variation in the circadian melatonin rhythm in humans: Entrained to local clock time, social time, light exposure or sun time?

That doesn’t mean that shorter days don’t affect us. There is a clear pattern of depression, bipolar and other mental health issues as the seasons change – Seasonality of brain function: role in psychiatric disordersWe can track a reduced amplitude in the serotonin or ‘feel-good’ system with shorter day length – Diurnal and seasonal variation of the brain serotonin system in healthy male subjects, and even a connection with your ability to read a map! (The hormone and hippocampal pathways that drive spatial awareness change with the seasons) – Seasonal variation in gonadal hormones, spatial cognition, and hippocampal attributes: More questions than answers.

Women tend to suffer more – up to 1.5 times more according to one study. We tend to eat more in the winter too, which doesn’t help! Interestingly, women’s mood tends to be more affected by cold, while men are more sensitive to cloudy days – An epidemiological study on gender differences in self-reported seasonal changes in mood and behaviour in a general population of northern Sweden.

The solution?

Given that you’re stuck with social, not sun time…

  1. Stick to a regular sleep-wake time and set your body clock with bright light. When it’s dark outside, artificial light will do. Current recommendation is 2 hours at 250 melanopic lux – Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults, or 500 lux of a standard domestic LED. Check your light levels with a free app like this one ( Power down at least three hours before bedtime (10 melanopic lux, or a low-wattage 2700k warm LED) Make sure the room is dark.
  2. Get outside as soon as it’s light. Windows block the infra-red and other wavelengths your cells need to stay on track. Even when it’s low in the sky, winter sun is good for you – Daily and Seasonal Variation in the Spectral Composition of Light Exposure in Humans.
  3. Reduce your risk of winter blues with Bright Light Therapy. A SAD lamp should deliver 10,000 lux for 30 minutes. A lower level for longer can work too: 2,500 lux for 1-2 hours.
  4. Avoid comfort eating if you can: snacking before bed makes it harder to sleep, and putting on the winter pounds won’t help – Effects of Dinner Timing on Sleep Stage Distribution and EEG Power Spectrum in Healthy Volunteers

People v. planet

A great question from Tim Bowes last week at Light23.  How do we balance the high light levels we need for the non-visual system and energy efficiency regulations?

You may be familiar with the recommendations from Brown et al’s seminal paper. To quote:

“Recommendations are intended to provide realistic targets that minimise inappropriate nonvisual responses in the sleep environment (melanopic EDI <1 lux) and reduce these so far as is practically possible presleep (3 hours before habitual sleep; melanopic EDI <10 lux) while maximising relevant effects across the intervening daytime hours (melanopic EDI >250 lux) ” – Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults.

This paper also tackles the issue of visual comfort and energy-efficiency head-on, pointing to the critical role of luminaire design, spectral composition, thoughtful layout and  surface reflectance.  It includes a set of calculations based on real-world office and classroom settings that  a model that can deliver up to three times more light to the non-visual system for the same energy input.

There does seem to be a dose-response relationship – it may be possible to reduce the amount of light you need if you deliver it for longer: in this study, the minimum amount of light needed to significantly suppress melatonin was 393 lux for 30 minutes, 366 for 60 minutes, 339 for 90 minutes and 285 lux for two hours – Human responses to bright light of different durations.

One plus to colder days – glorious sunsets!

This article explains how a phenomenon known as Rayleigh Scattering: different wavelengths are absorbed or bounced around by aerosols or tiny particles in the air. In the winter, the colder, drier air tends to let more wavelengths through – so we see those incredible colours in all their glory – The Science of Sunsets.


Need to top up your CPD points before Christmas?

Interested in Human-Centric Lighting?

I’m hosting an online CPD session on Friday 8th December from 12noon to 1:30pm UK time.

You’ll learn about the non-visual system, what the new measurements and standards mean and case studies from the office, healthcare and eduction sectors. Please register on the link here.


Light! Your Essential Survival Guide 

I’ve pulled together answers to the questions most people ask me about – how can they improve sleep, mood and memory. There’s a chapter on how the eye and brain work together, another on lighting technology and a simple buying guide. All the chapters have a QR code that takes you to the electronic version so you can track down the references directly.

It feels like work in progress and I would love your suggestions and comments about what you think is missing, what’s not clear – and what you think I’ve got wrong. I’m already planning a second edition for the Spring as so much new information comes through all the time – so would love to integrate any thoughts you might have: do get in touch – 

Here’s the link to order your copy.

Build a winter mindset

This fascinating paper found that simply focusing on the positive aspects of the winter – soft light, cosy time, festivals and social connections, can reduce your risk of feeling blue – Winter is coming: Wintertime mindset and wellbeing in Norway.

Get in touch!