The pineal gland releases melatonin, the hormone of darkness, and like the nick name given to it suggests, it works better with a lack of light...

In mammals, melatonin is secreted into the blood by the pineal gland in the brain. Known as the "hormone of darkness", it is secreted in darkness in both day-active (diurnal) and night-active (nocturnal) animals.
[link to en.wikipedia.org]

Dreaming

Some supplemental melatonin users report an increase in vivid dreaming. Extremely high doses of melatonin (50 mg) dramatically increased REM sleep time and dream activity in both people with and without narcolepsy.[49] Many psychoactive drugs, such as cannabis and lysergic acid diethylamide (LSD), increase melatonin synthesis.

Cancer

A systematic review of unblinded clinical trials involving a total of 643 cancer patients using melatonin found a reduced incidence of death.[74] Another clinical trial is due to be completed in 2012.[75] Melatonin levels at night are reduced to 50% by exposure to a low-level incandescent bulb for only 39 minutes, and it has been shown that women with the brightest bedrooms have an increased risk for breast cancer.[76] Reduced melatonin production has been proposed as a likely factor in the significantly higher cancer rates in night workers.

If you're worried about a lack of melatonin, just vaporize some cannabis or take some LSD.