How dark should your bedroom be for sleep?

Light is the main clock signal

The body clock uses light hitting the eye to decide what time it is. In the evening, as light fades, the brain starts releasing melatonin, the hormone that signals night to the rest of the body. Light pushes against that. In a study at Brigham and Women’s Hospital, adults who spent the evening in ordinary room light — under 200 lux, typical of a living room — had their melatonin rise later and the total time melatonin was elevated shorter than when the same people sat in dim light. Nothing about that light was unusual. It is what most of us have on until we go to bed.

Light during sleep matters too

It is tempting to think light only matters before you close your eyes. A 2022 experiment from Northwestern University suggests otherwise. Young healthy adults slept one night in very dim light and another night with an overhead light of about 100 lux on. Eyelids filter most of it, but not all. With the light on, heart rate stayed higher through the night and the next morning their bodies handled sugar less well, a sign of insulin resistance. The study was small and lasted a single night, so it shows a mechanism rather than a long-term risk, but it supports the common-sense advice to sleep in the dark.

How dark is dark enough

A practical test: turn everything off, wait a few minutes for your eyes to adjust, and hold your hand out at arm’s length. If you can see it clearly, light is getting in. The usual culprits are the edges of the curtains, street lights, a bright digital clock, chargers and standby LEDs, and light under the door.

Cheapest fixes first

Start with what costs nothing: turn the clock face away or dim it, put tape over standby lights, and close the door on hallway light. In the last hour before bed, switch from ceiling lights to a lamp or two, and keep screens dimmer.

Next, the window. Curtains labelled “blackout” vary a lot, and most light gets in round the sides and over the top rather than through the fabric. A wider rod mounted higher than the frame, or a shade that sits inside the frame, closes those gaps. If you rent or travel, a temporary paper shade or a portable blackout sheet is enough. We compare the three approaches on the blackout curtains page.

Finally, an eye mask covers what you cannot control: a partner reading, a hotel room, a long-haul flight, or daytime sleep after a night shift. A small experiment in a simulated hospital ward found that earplugs plus a mask improved sleep and melatonin under recorded noise and light. Contoured masks with cups that sit around the eyes avoid pressure on the eyelids; we compare several, along with earplugs, in sleep masks and earplugs.

The other half: light in the morning

The same clock that dislikes light at night needs it in the morning. Bright light soon after waking helps keep your sleep time stable and makes it easier to fall asleep the following night. So the aim is a dark room while you sleep and a bright one once you are up — open the curtains, or step outside for a few minutes. If early sunrise is waking you before the alarm, use our wake-up calculator to see whether moving your bedtime earlier would land you on a full sleep cycle instead, which is the approach our guide to waking at 5 am takes too.

Questions people ask

Is it bad to sleep with a night light?

A dim, warm-coloured light at floor level is a reasonable compromise if you need to get up safely. What the research flags is brighter light reaching your eyes for hours, such as an overhead light or a TV left on.

Do blackout curtains help you sleep longer in summer?

They help if early light is what wakes you. Sunrise in summer can come hours before your alarm, and light through closed eyelids is a wake-up signal. Closing the gaps around the curtain matters as much as the fabric.

Should the room be dark in the morning too?

Only until you want to be up. Morning light helps set your body clock, so open the curtains once the alarm goes off, or let a sunrise lamp brighten the room in the last half hour.

Do blue-light glasses help?

The trials so far are small and the evidence is weak. Lowering the overall brightness of the room in the evening is a surer bet than filtering one colour.