We were designed to sleep at night, when it is totally dark. One part of the human body’s cycle is melatonin production. Melatonin is a hormone that helps us to sleep. The pineal gland, which produces melatonin is very sensitive to light, and shuts down production if there is any light.
Melatonin has benefits to reduce insomnia and improve REM sleep cycle.
Even if you get up in the night and turn on the bathroom light, you will shut down production, probably for the rest of the night, because the pineal gland assumes that once it sees light, that it will be light for many more hours. Melatonin is only one part of the wake/sleep cycle that depends on light. The more time you spend in full daylight, during the day, and the more complete the darkness at night, the better you will sleep.
Do we have to fear to sleep in the dark? It's healthy indeed.