Light reaches the bed before you mean to wake, and a check at dawn will show where
If it is already light when you wake, daylight is reaching the bed before your alarm does, and one reading at the pillow at dawn will show where it gets in.
In short: Take a light reading at the pillow when morning twilight begins, then seal the gap where it shows before changing anything else.
What to check
| Check | What to do |
|---|---|
| What to measure | The light that falls on your face at the pillow, from the start of morning twilight until the time you mean to get up. |
| The instrument | A phone light-meter app is good enough to find where light gets in, but don't trust the exact number. In one comparison, deviations from the reference were in part extremely high (up to 113%) (source). Use the same phone for every reading. |
| The range that matters | For the sleep environment, the recommended maximum ambient melanopic EDI is 1 lux measured at the eye (Brown et al., 2022). Outdoors, at the edge of civil twilight, clear-sky illuminance is ∼3.5–2 lux (AMS Glossary), so it is already above that limit before sunrise. |
| Where to put it | Lay the phone flat on the pillow where your eyes rest, with the sensor facing the room. The guidance is set at the eye (Brown et al., 2022). |
| What to change first | The edges of the window covering: the top, the sides and the sill, where light gets around the fabric instead of through it. |
This page takes no readings of its own. The ranges come from the published sources linked in each row, and the reading is yours to take.
Why your answers lead here
Waking to light means the sky brightens before the sun is up. Civil twilight is the stretch between astronomical sunrise or sunset and the time when the sun's unrefracted center is at elevation h0 = -6°. Over a clear civil twilight, horizontal illuminance decreases from ∼585–410 lux to ∼3.5–2 lux, and at dawn the illuminance climbs back through those values in reverse (AMS Glossary). An uncovered east window can push a bedroom past the 1 lux limit while the sun is still below the horizon. In summer that starts before most alarms.
Closed eyelids cut this light down but do not remove it. One study measured eyelid transmittance and found that mean transmittance at 550 nm was approximately 0.5% and 0.4% at 490 nm (Bierman et al., 2011). A later paper adds that individual spectral transmittances can vary by one order of magnitude (Figueiro et al., 2012). Two people sharing a bed can respond very differently to one dawn.
The floor the bedroom is on changes what else to rule out. A top-floor room holds summer heat, so waking early there may come from warmth as well as light, and the guide to bedroom temperature for sleep covers that side. A middle-floor room usually keeps a steady temperature, which makes light the more likely cause. A ground-floor room runs cooler, but street lamps can light it all night, and a reading in the middle of the night, long before twilight, will tell street light apart from dawn.
What changes the answer
A winter answer means dawn comes late enough that light is an unlikely cause. Look at the heating and the air first, starting with ventilating a bedroom in winter without losing the heat. An all-year answer points to a constant source such as a street lamp, a standby light or a hallway light under the door. Real bedrooms often read higher than expected. In one cohort of people with bipolar disorder, a portable meter found nighttime illuminance with a median of 2.3 lux (0.3–9.6 lux) (Esaki et al., APPLE cohort).
Checking your own bed
Look up the time civil twilight begins in your town; the time of the end of civil twilight is often published in newspapers along with the time of sunrise and sunset. Set an alarm for it and leave the room as it would be on a normal night. Take one reading at the pillow, then hold the phone close to each edge of the curtain or blind. Start with the edge that reads highest. Change one thing at a time and take the same reading at the next dawn.
For covering the window itself, see how to choose blackout curtains for a bedroom and which blackout shades keep a bedroom dark. Both explain how light gets around the edges as well as through the fabric.