Sleep: A Dynamic Activity
Until the 1950s, most people thought of sleep as a passive part of our daily lives. We now know that our brains are very active during sleep. Moreover, sleep affects our daily activities and our physical and mental health in many ways that we are just beginning to understand.
The Stages of Sleep
Sleep in humans consists of two subtypes – rapid eye movement (REM) and non-REM (NREM) sleep; NREM sleep is further divided into subtypes, N1, N2 and N3. During sleep, we usually pass through 4 phases of sleep: stages N1, N2, N3, and REM sleep. These stages generally move in a cycle from stage N1 to REM sleep, then the cycle starts over again with stage N1. Adults spend almost half of our total sleep time in stage 2 sleep, about 20 percent in REM sleep, and the remaining 30 percent in the other stages. Infants, however, spend about half of their sleep time in REM sleep.
During stage N1, which is light sleep, we drift in and out of sleep and can be awakened easily. We have slow rolling eye movements and muscle activity decreases. People awakened from stage N1 sleep often remember parts of images. Many also experience sudden muscle contractions called hypnic jerks, and a sensation of starting to fall. These sudden movements are similar to the "jump" we make when startled. When we enter stage N2 sleep, our eye movements stop and our brain waves become less activated, with occasional bursts of rapid waves called sleep spindles.
In stage N3, extremely slow brain waves called delta waves begin to appear, which is why this stage is sometimes referred to as deep sleep. It is often very difficult to wake someone during stages N3. There is little or no eye movement or muscle activity. People awakened during deep sleep often feel groggy and disoriented for several minutes after they wake up.
REM Sleep
When we switch into REM sleep, our breathing becomes more rapid, irregular and shallow, our eyes move rapidly in various directions, and our limb muscles become temporarily paralyzed. Our heart rate and blood pressure increases. When people awaken during REM sleep, they often describe crazy tales — dreams.
The first REM sleep period usually occurs about 70 to 90 minutes after we fall asleep. An average sleep cycle from stage N1 to REM takes 90 to 110 minutes. The first sleep cycles each night contain shorter REM periods and longer periods of deep sleep. As the night goes on, REM sleep periods get longer while deep sleep decreases. By morning, people spend nearly all their sleep time in stages 1, 2, and REM.
Sleep Disrupters
Since sleep and wakefulness are influenced by different signals in the brain, foods and medicines that interact with these signals can affect whether we feel alert or drowsy and how well we sleep. Caffeinated drinks such as coffee and drugs such as diet pills and decongestants can cause insomnia, or an inability to sleep. Many antidepressants suppress REM sleep; some can be sedating, whereas others are more activating. Heavy smokers often sleep very lightly and have reduced amounts of REM sleep because nicotine is a stimulant. They also tend to wake up after three or four hours of sleep due to nicotine withdrawal. Many people who suffer from insomnia try to solve the problem with alcohol. While alcohol does help people fall asleep, it also robs them of REM and with prolonged use can lead to loss of the deeper, more restorative stages of sleep. Furthermore, after an evening of heavy drinking, the drop of blood alcohol levels in the middle of the night can produce insomnia from the withdrawal.
