Regardless of who deserves the most credit in its invention, before the incandescent bulb began to proliferate in the early twentieth century, human sleep schedules were largely governed by the Earth's day and night cycle. But once humans possessed the technology to ward off an appreciable chunk of nighttime, we soon extended our usable waking hours by an average of 13%. Some researchers believe that this modern convenience, credited with bringing the human race in from the dark, may also be responsible for numerous ills.
The root of the problem seems to be that unnatural light spawns unnatural behavior. Life on Earth evolved for millions of years in an environment with regular periods of daylight and darkness, and long ago set its clock to this 24-hour period. The planet's earliest organisms are thought to have adapted to replicate their DNA during the night to avoid the mutations caused by daytime's ultraviolet radiation; and later, as more complex organisms appeared, more complex motives for day and night behavior appeared. Since the length of days and night are predictable, organisms have evolved to take the maximum advantage from each. This physiological cycle is known as a circadian rhythm.
In mammals such as ourselves, these rhythms are governed by a group of brain cells located in the hypothalamus. Special light receptors found in the retina gather information on the length of the day and night, and pass the data on to the pineal gland in the brain, which then secretes the hormone melatonin during what it perceives to be nighttime. This system is slow to respond to change, which is what causes jet lag in travelers.
Once humans began to use artificial light to vary the length of the day, the average night's sleep decreased from about nine hours to about seven, and the amount of sleep began to vary considerably from one night to the next. This irregularity prevents one's circadian rhythm from settling into a pattern, and creates a state of perpetual semi-jet-lag. Our bodies' rhythms attempt to appropriately adjust our alertness, blood pressure, and such for particular times of day; but we often do things contrary to this cycle, and therein lies the problem.
A growing number of doctors believe that betraying our internal clocks is the source of a host of health problems. Once night falls, the body stays awake by activating the stress response, which in turn weakens the immune system. This is evidenced by the fact that individuals working graveyard shifts are more susceptible to stress, constipation, stomach ulcers, depression, and heart disease.
There is also considerable evidence to indicate that insufficient melatonin is linked to tumor formation, including childhood leukemia and breast cancer. One study found that melatonin slowed tumor growth by up to 70% in lab mice infected with human breast cancer cells, but when the mice were subjected to constant light, cancer growth accelerated. Interestingly, blind women have been found to be less at risk of breast cancer than sighted women, lending further evidence to the notion that melatonin plays a part in cancer reduction.
The evidence that artificial light strips us of some of our well-being is not conclusive, but it is concerning. If you must stay awake during the night, it may be prudent to leave the lights out, and allow the pineal gland to do its work. So far, studies on the effectiveness of artificially-administered melatonin have shown mixed results. In any case, a regular dose of darkness may be the best preventative measure against becoming a stressed-out, constipated, ulcer-riddled cancer patient.