Even the smallest amount of visible light can interfere with your sleep. You can help block out all light either by using blackout curtains to your bedroom or using a sleep mask.
The science: Melatonin is a chemical produced in the brain’s pineal gland at night time in order to regulate the body’s natural sleep/wake cycle. When there is even the slightest amount of light in your bedroom, your melatonin production is partially inhibited which interferes with your sleep. (Reiter, 1991).