The pattern of waking during the day when it is light and
sleeping at night when it is dark is a natural part of human life. Only
recently have scientists begun to understand the alternating cycle of sleep and
waking, and how it is related to daylight and darkness.
A key factor in how human sleep is regulated is exposure to
light or to darkness. Exposure to light stimulates a nerve pathway from the
retina in the eye to an area in the brain called the hypothalamus. There, a
special center called the suprachiasmatic nucleus (SCN) initiates signals to
other parts of the brain that control hormones, body temperature and other
functions that play a role in making us feel sleepy or wide awake.
The SCN works like a clock that sets off a regulated pattern
of activities that affect the entire body. Once exposed to the first light each
day, the clock in the SCN begins performing functions like raising body
temperature and releasing stimulating hormones like cortisol. The SCN also
delays the release of other hormones like melatonin, which is associated with
sleep onset, until many hours later when darkness arrives.
What is Melatonin?
Melatonin, a hormone produced in the brain’s pineal gland,
is responsible for the regulation of the body clock in each individual.
Interestingly, the release of this hormone is largely controlled by exposure to
natural light, or a lack thereof.
Melatonin production is first triggered in the evening, but
the hormone continues to be released throughout the hours of darkness that
follow (the conventional sleeping period). Levels of melatonin then drop with
the breaking daylight and its production is suppressed until the next evening.
Due to its dependence on a person being in a dark environment, melatonin is
often referred to as the “hormone of darkness”.
It is precisely this link between darkness and melatonin,
which informs advice to keep your bedroom dark and free from light-emitting
electronics. Research has shown, for example, that both melatonin production
and deep sleep phases are better maintained in the dark.
Melatonin and electric lighting
The introduction of electric light in the 19th century is
often described as having had a negative influence on sleep. Before we had
light bulbs and lamps in our homes, our circadian rhythms were dictated by
natural light. People would wake with the first light of day and retire to bed
early in the evening as the darkness fell.
The invention of artificial light however allowed people to
make use of the evening after the sun went down. This change to our daily
schedules is thought to have brought about changes in our sleep schedule, pushing
our bedtime ever later.
Electric lighting continues to impact our sleep, not least
because we are now free to work late into the evening and even throughout the
night.
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