As the most powerful zeitgeber , light has a direct effect on sleep. Light influences our internal clock through the light-sensitive cells in the retina of the eyes. Since electricity and the lightbulb were invented, people have been going to bed later. Midnight is no longer the middle of the night, but bedtime for many people. Artificial light has also made night shifts possible. Such work patterns mean less exposure to natural sunlight during the day, but lots of artificial light at nighttime.
Every time we turn on a light, we are inadvertently taking a drug that affects how we will sleep.
— Charles CzeislerCC Charles Czeisler , quoted by Ekirch in The American Historical Review
Both exposure to artificial light and lack of exposure to sunlight affects natural circadian rhythms . Artificial light is any light source that is produced by electrical means. Too much exposure to artificial light before bed hinders sleep onset and quality. Light affects many systems in the human body. Its most obvious effect is the suppression of melatonin . The more bright light we get in the morning, the more awake and alert we become. Darkness boosts our melatonin levels, making us naturally sleepy.
Linda Geddes 
LG
Linda Geddes
decided to live for a few weeks without artificial light and strictly followed getting enough light during the day and limiting it during the night. She reported being significantly sleepier in the evenings as her body fell into the habit of a proper sleep/wake cycle. Others who conducted the same experiment found themselves sleeping at least an hour earlier and slept around two hours more.
Getting enough light during the day maintains our natural circadian rhythm, which should make it easy to fall asleep in a dark room at night.