Using a wearable sensor to quantify the benefits of light exposure
The previous article looked at light exposure and its benefits to both strengthen circadian rhythms and boost energy. Today we will explore the process of monitoring light exposure, determining the quality of exposure through the day and the potential effect on alertness and mood.
Both the intensity, temperature and direction of light into our eyes has a dramatic impact on alertness and mood through the day, and quality of sleep. We should safely get exposure to fairly intense (1000–10,000lux), blue light in the morning to kick off circadian rhythms, and through the day where possible to maintain alertness. As we transition to evening, we should expose ourselves to natural sunlight as the sun sets to create some buffer to later light exposure, and balance any circadian shift from the morning. Finally, in the 2–3 hours before bed, we should limit light exposure (especially overhead), to allow the natural rise of melatonin in preparation for sleep.
THE LYS SENSOR AND APP
Through research online, I came across a wonderful sensor and app called LYS. This company has produced a “button like” sensor, that you clip onto clothing close to your eyes, near top button of a shirt or the top edge of jumper or t shirt. It continuously monitors both the light intensity and light temperature you are exposed to. It plots this data within an app, and provides a score of 0–100% (100% being good) for various sections of the day, and indicates the wavelength of light through a varying colour profile of the bars.
The app also provides a live feature that rates your current light source as SLEEPY / DROWSY / ENERGISED based on various thresholds. You can use the button or your phone’s light sensor to detect the current levels.
Note I approximated the “Light Stimulus” reading from 0–100 to a Lux light reading using an iPhone light meter app with a paper diffuser to improve accuracy. See my rough conversion below. It seems that actually quite low intensities (2000 lux) were enough to read 100%. Interestingly a study in 2011 showed that duration is more important than intensity, so maybe LYS believes that 2000lux as received at the eyes, is high enough to benefit.
DAYTIME LIGHT EXPOSURE — A TYPICAL DAY & GOOD HABITS
The Lys app breaks down the day into 4 sections based on my waking time of 7AM and sleeping time of 10:30PM:
1. Morning 7AM-10AM: During this period we should be maximising our light intake, especially to blue light. Using the app, I found this easy to achieve 100%, through my morning routine of a 15 minute walk and 15–30 minute stretch and meditate outside. If the morning is particularly cloudy / dark I use a 10,000 lux SAD light.
2. Daytime 10AM-4PM: During this period getting plenty of light is good to keep melatonin levels low and keep you alert. Reaching 100% here was easy to achieve through either an hour or more outside, being near a window on a sunny day, or in good office lighting all day (40–50 “Light Stimulus” or 500+ lux). Being inside a typical house or low lit office was not enough to keep the app at 100%, or me alert.
3. Evening 4–8PM: Exposure to the sunset and natural light with a natural shift towards red temperature will help to inform the body that daytime is coming to an end, and will help buffer the negative effects of getting light later. My intake levels naturally lowered here.
4. Night 8–10:30PM: During this period all light exposure should be minimised, but especially blue wavelength light. All my screens and smart lights in the house shift to low intensity, red temperature light from 8PM, I turn on lower lighting where possible, and ensure no screens within 1–2 hours of bed. This always scored me 100%, where lower light intake means a higher score.
SUMMARY
Overall this is a great way to monitor light exposure through the day, and ensure that daily habits lead to good light hygiene. Having only paid for the basic software subscription, I am only receiving the subjective levels of “Light Stimulus” and corresponding scores out of 100% for each portion of the day, but you can pay to unlock all data.
Next steps with this for me, would be to either pay for the raw data or better record the intensity in Lux and temperature in Kelvin, and compare this to a subjective “alertness” score to better understand the effect of intensity, exposure time, and temperature against a perceived alertness / energy.
As always thanks for reading, and please let me know if you’ve experimented with light intake and your findings.
