Mainstream sleep advice ignores this (and that's a huge mistake)....
A missing piece of sleep biology that not enough people understand
Most sleep advice treats bedtime as a single event: you turn the lights off, the room goes dark, melatonin rises, and sleep follows. That framing sounds logical, but it doesn’t reflect how the circadian system actually works.
If you have been in my community for a while - you know I have specific recommendations around the 3 hours before bedtime.....but today I am going to break down the science of WHY I give such specific advice around blue light AND lux.....
Your circadian system doesn’t respond to light like an on & off switch. It responds to light as a cumulative signal, especially in the evening.
Even if you turn the lights off right before bed and sleep in total darkness, your circadian timing can remain delayed depending on the type, intensity, and duration of light exposure earlier in the evening.
In other words - watching TV - scrolling on your phone or simply keeping bright overhead lights on (like the majority of people do) up until the moment you actually go to bed -could be keeping you awake!
The science
Your eyes contain a small group of cells called intrinsically photosensitive retinal ganglion cells, or ipRGCs. These cells don’t exist for vision - their job is to tell your brain what time of day it is. Unlike rods and cones, which respond quickly and shut off fast, ipRGCs integrate light over time and are slow to deactivate, particularly when exposed to bright or blue-rich light.
In practical terms, that means evening exposure to indoor lighting and screens can continue to influence circadian signaling even after the lights go out. This is why people can struggle with falling asleep, staying asleep, or waking up feeling restored despite sleeping in complete darkness. It’s often not a bedtime problem - it’s a signaling problem from earlier in the night.
The good news is that you don’t need to micromanage your life to work with this biology. You just need clearer boundaries around evening light.
In the three hours leading up to bed, aim to block blue light & keep ambient light levels at or below 10 lux. In the final hour before sleep, bring that down to under 1 lux whenever possible. Your sleep environment itself should also remain under 1 lux overnight.
This matters because those lower light levels reduce stimulation of the melanopsin pathway, allowing melatonin to rise on time and the nervous system to transition into nighttime physiology.
If you want to take the guesswork out of this, this is exactly what I built MyCircadianApp for. You can use it to measure lux in your home, check your evening light exposure, and make simple adjustments without overthinking it. (download here & use code SARAHK for a free trial)
Most people are shocked by how bright their evenings actually are (even when they wear blue blockers), and how small changes make a real difference.
Forward this to a friend or family member if you learned something new today!
