Answer:
Sleep in the Real World
The internal mechanisms that regulate our almost ceaseless cycles of sleep and wakefulness make up a remarkable system. However, a variety of internal and external factors can dramatically influence the balance of this sleep-wake system.
Changes in the structure and function of the brain during development can have profound, if gradual, effects on sleep patterns. The amount of sleep we obtain generally decreases and becomes more fragmented throughout our lifespan. These and other variations associated with age are covered at length in the essay Changes in Sleep with Age.
Other factors that affect sleep include stress and many medical conditions, especially those that cause chronic pain or other discomfort. External factors, such as what we eat and drink, the medications we take, and the environment in which we sleep can also greatly affect the quantity and quality of our sleep. In general, all of these factors tend to increase the number of awakenings and limit the depth of sleep.
Light's Effect
Light exposure can cause our biological clock to advance or delay, which affects our sleep and wake cycle.
Light is one of the most important external factors that can affect sleep. It does so both directly, by making it difficult for people to fall asleep, and indirectly, by influencing the timing of our internal clock and thereby affecting our preferred time to sleep.
Light influences our internal clock through specialized "light sensitive" cells in the retina of our eyes. These cells, which occupy the same space as the rods and cones that make vision possible, tell the brain whether it is daytime or nighttime, and our sleep patterns are set accordingly.
Due to the invention of the electric lightbulb in the late 19th century, we are now exposed to much more light at night than we had been exposed to throughout our evolution. This relatively new pattern of light exposure is almost certain to have affected our patterns of sleep. Exposure to light in the late evening tends to delay the phase of our internal clock and lead us to prefer later sleep times. Exposure to light in the middle of the night can have more unpredictable effects, but can certainly be enough to cause our internal clock to be reset, and may make it difficult to return to sleep.
Jet Lag and Shift Work
Normally, light serves to set our internal clock to the appropriate time. However, problems can occur when our exposure to light changes due to a shift in work schedule or travel across time zones. Under normal conditions, our internal clock strongly influences our ability to sleep at various times over the course of a 24-hour period, as well as which sleep stages we experience when we do sleep.
Long-distance travelers experience “jet lag” as their internal clock adjusts to the new day-night cycle.
Individuals who travel across time zones or work the night shift typically have two symptoms. One is insomnia when they are trying to sleep outside of their internal phase, and the other is excessive sleepiness during the time when their internal clock says that they should be asleep. Half of all night shift workers regularly report nodding off and falling asleep when they are at work. This should be seen as an important concern both for individuals and society, given that airline pilots, air traffic controllers, physicians, nurses, police, and other public safety workers are all employed in professions in which peak functioning during a night shift may be critical.
The effects of shift work and jet lag on sleep are covered in much greater detail in Jet Lag and Shift Work and You and Your Biological Clock.
Pain, Anxiety, and Other Medical Conditions
A wide range of medical and psychological conditions can have an impact on the structure and distribution of sleep. These conditions include chronic pain from arthritis and other medical conditions, discomfort caused by gastroesophageal reflux disease, pre-menstrual syndrome, and many others. Like many other sleep disruptions, pain and discomfort tend to limit the depth of sleep and allow only brief episodes of sleep between awakenings.
Individuals of all ages who experience stress, anxiety, and depression tend to find it more difficult to fall asleep, and when they do, sleep tends to be light and includes more REM sleep and less deep sleep. This is likely because our bodies are programmed to respond to stressful and potentially dangerous situations by waking up. Stress, even that caused by daily concerns, can stimulate this arousal response and make restful sleep more difficult to achieve.
Medications and Other Substances
Many common chemicals affect both quantity and quality of sleep. These include caffeine, alcohol, nicotine, and antihistamines, as well as prescription medications including beta blockers, alpha blockers, and antidepressants.
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