Why can't you see anything for a while
after walking into a dark room?
It actually takes about 30 minutes for your eyes to fully adjust to darkness. Your pupils finish opening in just a few seconds. What takes so long is the time it takes for "dark-place" cells at the back of your eye to rebuild the pigment that light has broken down.
You walk in from a bright afternoon outside into a cinema where the film has already started. Your feet, the seats, almost everything is pitch black.
But a few minutes after you sit down, you start to faintly make out the face of the person next to you, or the step in the aisle. After ten or twenty minutes, you can even see things that were invisible when you first walked in.
The room's brightness hasn't changed. What's changed is you — or rather, your eyes. So what's actually happening inside them?
There are just two reasons it takes so long
The retina at the back of your eye has two kinds of cells: ones that see color in bright light, and ones that pick up light in the dark. Right after being in a bright place, the dark-place cells aren't working yet.
The dark-place cells sense light using a pigment that breaks down when hit by light. In bright places, most of this pigment has already broken down. Rebuilding it takes several tens of minutes.
People often assume "I can see because my pupils are opening." But pupils finish opening within seconds, and that only accounts for a small part of the improvement in visibility. The real story is in the retina.
Around the 10-minute mark, your eye's "duty cells" swap over
The retina has two kinds of light-sensing cells. One is the cone. It works in bright light and distinguishes color. The other is the rod. It can't tell color, but it responds to far weaker light than cones can.
When you enter a dark place, the cones adapt within a few minutes — and that's it. They can't see any dimmer than that. Meanwhile, the rods are slowly getting ready. Look at Figure 1. About 7 to 10 minutes after entering, the rods overtake the cones. From that point on, the "gradually seeing more" is the rods' doing.
The reason rods take so long to get ready is a light-sensing pigment called rhodopsin. Every time rhodopsin absorbs a photon, it changes shape and becomes unusable. In bright daylight, most of it sits in this unusable state. Turning it back is a slow process that happens bit by bit in the dark. It's thought to be almost fully restored only after about 30 minutes.
In the dark, staring straight at it makes it vanish
Rods have another interesting property. The very center of the retina — where whatever you're staring at gets projected — has almost no rods. That spot is packed instead with cones, which see color and fine detail (Figure 2).
That's why, under a dark night sky, if you stare straight at a faint star, it fades away. Look slightly to the side, and it reappears. Stargazers call this trick "averted vision" and use it deliberately. When searching for something in a dark room too, looking a little off to the side works better than looking straight at it.
Rods barely sense red light. So under red light, rhodopsin stays largely intact. That's why stargazers use red flashlights. It's also why, at dusk, red flowers fade to a dark blur first while blue flowers stay bright — a sign that control is shifting to the rods.
Going from a dark place into a bright one, the glare settles down within seconds to about a minute. That's because "breaking down" the pigment happens instantly once light hits it, while "rebuilding" it takes work. Tunnel entrances are lit brighter than the interior for this same reason — to make up for your eyes' lag as you enter.
A useful thing to know
At night, if you look at a bright phone screen and then walk down a dark hallway or staircase, your footing suddenly becomes hard to make out. That's because the light from the screen breaks down the rods' pigment all over again. In the first few minutes after entering a dark place, you tend to feel like you can see when you actually can't. On uneven ground, use a light to check your footing.
The speed of dark adaptation is thought to slow down with age. Rhodopsin is made from vitamin A, and it's known that a severe shortage of it can cause "night blindness," a condition where dark vision fails.
Summary
The time it takes to start seeing in the dark is the time it takes for control to pass from cones to rods, and for the rods to rebuild the pigment that light broke down. The pupils only need the first few seconds; the remaining 30 minutes is retinal chemistry at work.
Your eyes don't simply "get used to" darkness —
they rebuild the parts needed for it.
For how strong light can hide weak light, see "Why do windows turn into mirrors at night?" and "Why does a pedestrian crossing the road at night suddenly disappear from view?"; for why your body sways in a dark room, see "Why do you sway so much walking through a dark room at night?" For animal eyes that glow in the dark, see "Why do animals' eyes glow at night?"
- At night, in a lit room, cover one eye firmly with your palm and spend about 20 minutes like that (reading a book is fine).
- Turn off the lights and remove your hand. Close each eye in turn and compare how the dark room looks through each one.
- Only the eye that was covered should clearly make out the shapes of furniture. Turn the lights back on, and now only that eye will feel dazzled.
This shows that dark adaptation happens "per eye." It's proof the change occurs in the retina, not the brain. In your darkened room, clear away anything you might bump into first, and try this while seated.
Want to know more? — Terms, formulas, and textbook connectionsWe label which level each part belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Biology"
- HS+High-school advanced content, or textbook sidebar material
- UnivNot covered in high school — university-level specialist content (visual physiology, biochemistry)
- ResearchNot even settled fact at university level — something researchers are still investigating
MSTerms: this phenomenon has a name
- Dark adaptation: the gradual rise in eye sensitivity when moving from a bright place to a dark one.
- Light adaptation: the reverse — glare settling down when you step from a dark place into a bright one. This finishes much faster than dark adaptation.
- Cones and rods: the two kinds of light-receiving cells in the retina. Cones handle bright light and color; rods handle dark places. There are thought to be about 6 million cones and about 120 million rods.
MSHSCheck with a formula: how much does pupil opening alone help you see?
The pupil is thought to be about 2 millimeters across in bright places and open up to about 8 millimeters in the dark. The amount of light entering is proportional to the pupil's area. Let's compare that with the overall rise in sensitivity during dark adaptation.
| Pupil diameter in bright places (approx.) | 2 mm |
| Pupil diameter in dark places (approx.) | 8 mm |
| Overall sensitivity increase during dark adaptation (approx.) | around 100,000-fold (said to vary greatly by conditions) |
| Ratio of diameters | 8 ÷ 2 = 4 |
| Ratio of areas (diameter ratio squared) | 4 × 4 = 16 |
| Factor handled by the retina (not the pupil) | 100000 ÷ 16 = 6250 |
The pupil opening only lets in about 16 times more light. The remaining several-thousand-fold increase comes from the rods switching on and their pigment being restored inside the retina. "I can see because my pupils open" is only a small part of the whole story. In symbols: the amount of light entering is proportional to the diameter d squared (any consistent units work).
HSHS+Rhodopsin's "breakdown" and "regeneration"
HSRhodopsin, the rods' pigment, is made of the protein opsin bound to retinal, which is derived from vitamin A. When it absorbs light, retinal changes shape and detaches from opsin. This triggers the electrical signal that lets you sense light.
HS+Once detached, retinal can't be reused as is. It's carried to a layer of cells called the retinal pigment epithelium, deep in the retina, restored to its original shape, and sent back to the rods. This round trip is thought to be the main reason dark adaptation is so slow. Cones, by contrast, get help from other retinal cells to regenerate faster, which is why they adapt so quickly.
UnivThe kink in the sensitivity curve, and the Purkinje shift
If you keep measuring the "dimmest light still visible" throughout dark adaptation, you get a curve that kinks partway through, as in Figure 1. That kink is the "handover between cones and rods" — if you shine light only on the very center of the retina, the kink disappears and only the cone curve remains. Cones are also thought to respond best to yellow-green light (around 555 nanometers), and rods to blue-green light (around 500 nanometers). When control shifts, the color that looks brightest shifts toward blue. This is called the Purkinje shift, and it's why red flowers fade to dark at dusk.
ResearchWhat's still not fully understood
- How few photons does it take to "see" something? A 1942 experiment found that people could report "seeing" a flash with only a handful of photons, and a single rod is thought to be able to respond to a single photon. Where the brain draws the line between a real signal and noise is still being studied.
- Why does adaptation slow down with age? Delayed regeneration in the pigment epithelium is suspected. Research is also underway into using dark adaptation as an early clue for eye disease.
- Did pirates wear eyepatches to stay dark-adapted? A popular theory holds it was to move between a sunlit deck and a dark ship's hold. It makes logical sense, but no solid record has been found that pirates actually used it this way.
In other words, everything in this article is "the best explanation we currently have." The figures for time and magnification in particular are rough guides that vary a great deal with light intensity, age, and measurement method.
Textbook connections (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | Science Year 2, "Stimuli and response (structure of the eye)" | Retina and pupil function, pupil area calculation |
| HS | Biology, "Animal response and behavior (photoreceptor cells)" | Cones and rods, rhodopsin |
| HS+ | Biology advanced / reference book sidebar | Retinal regeneration and the pigment epithelium |
| Univ | Visual physiology / biochemistry | Kink in the dark-adaptation curve, Purkinje shift |
| Research | Vision science / ophthalmology | Photon-count threshold, age-related slowing |
| ― | Everyday connections | Cinemas, dark staircases, red lights for stargazing, tunnel lighting |
- Hecht, S., Shlaer, S., & Pirenne, M. H. (1942). Energy, quanta, and vision. Journal of General Physiology, 25(6), 819–840.
- Lamb, T. D., & Pugh, E. N. (2004). Dark adaptation and the retinoid cycle of vision. Progress in Retinal and Eye Research, 23(3), 307–380.
- Webvision: The Organization of the Retina and Visual System (University of Utah)
- High-school biology textbooks and illustrated reference guides (photoreceptor cells and rhodopsin unit)
※This article is a general-audience science explainer. The figures given are rough estimates meant to help explain the mechanism. How well people see in the dark varies from person to person; if your night vision suddenly worsens, please consult an ophthalmologist.