One study documented depressed melatonin in connection with B-12 intakes of 3 mg per day. A bright indoor environment may provide 400 lux. On the other hand, outdoor light is incredibly more intense than the artificial variety. Outdoor light can reach 3000 lux on a bright sunny day. Researchers in Finland found that rats who were exposed to natural daylight had significantly higher evening melatonin levels than another group of rats who had an equal amount of exposure to artificial light.
Evening light suppresses melatonin production in humans.
In Western nations, the habit of staying up late at night is generally associated with significant amounts of light exposure during those hours. From the standpoint of melatonin production, it would be better to go to bed early. Research suggests that controlling stress can improve melatonin production. It cut melatonin production in half for 6 hours. Alcohol also depresses melatonin levels.