Until the middle of the 20th century, people generally thought of sleep as a passive activity in their lives. Since then, we have been much enlightened in this all-too-critical area. For instance, we now realize that our brains are highly active while we sleep. As well, we know that sleep quality can dramatically affect our ability to function, as well as our physical and mental health in ways that we are just now beginning to fully understand.
Chemicals that signal our nervous system are called neurotransmitters, and they control whether we are awake or asleep by working on different sets of nerve cells (neurons) in the brain. Neurons found in the brainstem, which attaches the brain to the spinal cord, produce neurotransmitters (like nor-epinephrine and serotonin), which keep certain areas of the brain active while we are awake. Other neurons found at the base of the brain begin sending signs when we are asleep. These neurons seem to "shut off" those signals that we know keep us awake. Recent research also points to a chemical called adenosine, which builds up in the bloodstream while we are awake and causes drowsiness, then slowly breaks down as we sleep.
While sleeping, we typically go through five different phases. These are simply known as stages 1, 2, 3, 4, and then REM (rapid eye movement) sleep. These stages advance in a cycle from stage 1 all the way to REM sleep, and then the cycle begins again with stage 1. Most adults spend about 50 percent of the total time asleep in stage 2, and only about 20 percent in REM sleep, with the other 30 percent spent in various other stages. Interestingly, infants spend close to 50 percent of their sleep in REM sleep.
While in stage 1, or light sleep, we tend to drift in and out of sleep, and are easily awakened. Muscle activity slows down considerably and the eyes move very little and very slowly. Those who wake up during stage 1 sleep frequently remember disjointed or fragmented visual images. Many people can also experience sudden and dramatic muscle contractions, which are called hypnic myoclonia. This event is frequently preceded by a sudden sensation of falling. These movements resemble the reactions we exhibit when startled.
As we move into stage 2 sleep, the eyes tend to stop moving and our brain waves (which are fluctuations of electrical activity) become much slower, with occasional rapid waves known as sleep spindles. Once we get into stage 3 sleep, extremely slow brain waves (known as delta waves) begin to appear, mixed in with other smaller and faster waves. By the time we reach stage 4, the brain shows delta waves almost exclusively. During stages 3 and 4 (known as deep sleep), it’s extremely hard to wake someone. During this phase, there is no muscle activity and no eye movement. Those who are awakened during deep sleep tend not to adjust immediately and often feel groggy and disoriented for several minutes.
As we transition into REM sleep, our breathing becomes much more rapid, shallow and irregular. Also, our eyes twitch rapidly in random directions and our limbs become temporarily paralyzed. Our heart rate also increases and our blood pressure elevates some. People who are awakened during REM sleep sometimes claim strange, nonsensical dreams.
The first period of REM sleep normally occurs at about 70 - 90 minutes after we fall asleep, and a full sleep cycle takes an average of about 90-110 minutes. The first nightly sleep cycles have fairly short durations of REM and longer durations of deep sleep. As sleep progresses, REM periods increase in length and deep sleep decreases. Most mornings, people spend almost their entire sleep time in stage 1, 2, and REM sleep.
People who are awakened after sleeping for more than about 5 minutes are usually unable to remember the last few minutes before they fell asleep. This is known as a sleep-related form of amnesia and is the reason people tend to forget phone calls or conversations they've had during the middle of the night. It’s also the most common reason why we sometimes forget our alarm clock ringing in the morning, when we go right back to sleep after turning them off, and this is the reason for the snooze feature found on most alarm clocks.
Because sleeping and awake times are heavily influenced by various neurotransmitter signals to the brain, foods and medicines that can alter the balance of such signals may affect whether we feel awake or sleepy as well as affect how well we do sleep. Drinks with caffeine such as coffee or colas along with diet pills and decongestants stimulate certain parts of the brain and sometimes cause insomnia. A number of antidepressants also curb the REM sleep stage. Those who are under the effect of anesthesia or even in a coma are sometimes assumed to be asleep. However, people in these circumstances can’t be woken up, and don’t produce those active brain wave patterns typically seen during normal sleep stages. Instead, those brain waves are very weak, slow and sometimes even undetectable.
Those who are considered heavy smokers (one pack or more per day) typically sleep lightly and experience lowered amounts of REM sleep. They also show a tendency to wake up after 3 or 4 hours of sleep because of associated nicotine withdrawals. Many of those who suffer from insomnia have been known to try and resolve the problem with alcohol, but while alcohol can help people fall into a light sleep, it also prevents REM stage sleep as well as the deeper, more recuperative stages of sleep. To the contrary, alcohol tends to keep them in the lighter stages of sleep, allowing them to be awakened easily, thus providing an ineffective and unhealthy sleep pattern.
People also lose some of their ability to regulate body temperature during REM stage sleep, so unusually hot or cold temperatures in the sleep environment can sometimes interrupt this stage of sleep. If REM sleep is disturbed one night, the normal sleep cycle progression is affected the next time we fall asleep. When this occurs, we usually go directly into the REM sleep stage and experience extended periods of REM sleep until we have caught up on this critical sleep stage.
The conclusion we have clearly reached is that an undisturbed and normal sleep pattern should be maintained if at all possible. Avoiding unwanted influences in the sleep environment is of paramount importance in achieving a healthy sleep pattern.
(Sources: American Sleep Association, National Institute of Neurological Disorders and Stroke, The National Institutes for Health and Bedbug.com)
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Part 1: Sleep Basics - Why Do We Need Sleep (this article)
Part 2: How Much Sleep is Enough Sleep
Part 3: What are the Benefits of Sleep
Part 4: Importance of REM Stage Sleep and Dreaming
Part 5: What are Circadian Rhythms
Part 6: Disease Associated with Sleep
Part 7: Sleep Disorders
Part 8: Clean Sleep Environment