Short answer: reading on paper by a weak, warm reading light gives by far the smallest circadian signal. An e-reader without its own lighting comes close. A backlit screen is the least favourable option, mainly because the light sits right in your line of sight at close range, not just because it contains blue.
The background is in the article on which light disturbs your sleep and which does not.
Why reading is a tricky case
With reading, three unfavourable factors come together. You need quite a bit of light to read comfortably. You do it just before going to sleep, exactly in the window when your clock is most sensitive. And you usually do it for half an hour to an hour, so in the range where melanopsin dominates the response.
That is also why “just use a warm lamp” helps less here than people hope: the amount of light you need to read comfortably stays the same.
Paper, e-reader or screen
| Way of reading | Where the light comes from | Verdict |
|---|---|---|
| Paper with a reading light | Reflected off the page, lamp out of your line of sight | Best: you control brightness, colour and direction |
| E-ink without lighting | External lamp, same principle as paper | Almost the same as paper |
| E-ink with front light | Weak light along the screen, can be set warm | Reasonable, if set low and warm |
| Tablet or phone | Backlight straight in your line of sight | Least favourable |
With paper you see light reflected off the page; a good part of the light goes elsewhere and the lamp itself is beside you. With a screen you look straight into the light source, thirty centimetres from your eyes. At the same image brightness, more light then reaches your retina.
What night mode does and does not do
Night mode shifts the colour of the screen to warm. That lowers the melanopic content of the light, and that is measurably useful.
What night mode usually does not do is lower the brightness. And that is where a big part of the gain is. The study by Schöllhorn and colleagues (Communications Biology, 2023) kept brightness and perceived colour the same and only varied the melanopic content: the effects depended on the dose, so the more light, the bigger the response. A warm colour at full brightness leaves that dial untouched.
In practice: set the brightness as low as the room allows, and use night mode on top of that. Two dials, not one.
A good reading setup
- Light only the page. A directed reading light with a narrow beam gives less total light than a ceiling lamp that lights up the whole room. A red reading light with a book clip aims the light at the page.
- Keep the lamp out of your line of sight. From the side or from behind over your shoulder, so you never look into the light source.
- Warm and dimmable. Warm makes a measurable difference, and dimmable makes more.
- Watch out for flicker. Cheap dimmers introduce flicker you do not consciously see but that makes reading tiring. IEEE 1789-2015 sets limits for this; ask for the flicker percentage.
- Look at the whole hour. Any light before and after reading (bathroom, hallway, phone) simply counts too.
Why filtering blue alone is not enough
“Filter out the blue and you are fine” is too simple. The PNAS study by St Hilaire and colleagues (2022) found that peak sensitivity for melatonin suppression in the first quarter of an exposure was at 441 nm, with a second peak around 550 nm, the range you see as yellow green. Only with longer exposure does the peak shift to 485 nm, where melanopsin sits.
For an hour of reading you are mostly in the melanopsin range, where filtering blue helps most. But for the short, bright moments around it (the bathroom light, a quick look at your phone) your whole visual system takes part, and only less light helps.
Sources
- Schöllhorn et al., Melanopic irradiance defines the impact of evening display light on sleep latency, melatonin and alertness, Communications Biology 6:228, 2023
- St Hilaire et al., The spectral sensitivity of human circadian phase resetting and melatonin suppression to light changes dynamically with light duration, PNAS, 2022
- Jiexiu Zhao, PhD et al., Red Light and the Sleep Quality and Endurance Performance of Chinese Female Basketball Players NATA, 2012
- Shadab A Rahman et al., The effects of spectral tuning of evening ambient light on melatonin suppression, alertness and sleep ScienceDirect, 2017
