In a survey by the National Sleep Foundation, 73% of respondents said that a dark room was important to getting a good night’s sleep. And scientists agree. Recently, researchers found that hamsters exposed to dim lights (equal to the illumination from a television screen) nightly for eight weeks showed more symptoms of depression than hamsters who slept in the dark. And an eight-week study on mice showed that those exposed to dim light at night gained about 50% more weight than mice keep in nighttime darkness—even though food intake and activity levels were the same in both groups.

Are humans vulnerable to the same effects? We very well might be. We called Meir H. Kryger, MD, director of sleep medicine research and education at Gaylord Sleep Medicine of Gaylord Hospital in Wallingford, Connecticut, and author of A Woman’s Guide to Sleep Disorders. He explained that, when it’s dark, the pineal gland in the brain produces the hormone melatonin, which makes you sleepier and also affects your circadian rhythm (internal clock). Excess light at night inhibits melatonin production, which can wreak havoc with your sleep/wake cycle. This can be particularly problematic for women, given that we are already twice as likely as men to suffer from insomnia. What’s more: Melatonin also plays a role in regulating blood pressure and blood glucose levels—making it even more important to keep your bedroom sufficiently dark.

How dark is dark enough? You should not be able to see details in your room at night even after your eyes habituate to the darkness, Dr. Kryger said. If your room is too bright…

Get light-blocking window blinds or shades, if necessary, to keep out streetlights and other ambient light… or at least wear a sleep mask.

Keep the hall light off. If other household members are still awake when you go to bed, shut your door.

Replace your illuminated alarm clock. Choose one with a built-in feature that automatically dims the clock face at night or that illuminates only when you press a button. You might even try getting rid of your alarm clock! While this seems like a shocking proposal to those of us who fear that we’ll sleep in until noon, Dr. Kryger said that most people do not actually need an alarm clock because they wake up before it goes off.

Turn off the tube. If you or your partner cannot get to sleep unless the television is on, there’s help. A psychologist trained in cognitive behavioral therapy can retrain a person to fall asleep without the television. Alternative: Invest in a television with a built-in timer that turns itself off and program it to do so at a time after which you would typically be asleep.

Check for other sources of light. Lie in your bed and look around. Is light coming from a computer, house phone, cell phone, cable box, alarm keypad or any other device? Unplug it, block its glow or move the device to another room. If you need a night-light to find your way to the bathroom safely, be sure to use one that is very dim (try an energy-efficient LED night-light) and place it where its slight illumination will not disturb your slumber.