Although I’ve already written a brief post on the sleep cycle there was a lot more that could be said on the subject. I believe it is helpful to understand how we sleep, as all other information about how to improve sleep must be based on it.
There are two major types of sleep: Non-REM, or quiet sleep; and REM, or dreaming sleep. REM, you may recall, stands for ‘rapid eye movement’ and refers to the rapid movements the eyes make while in that stage of sleep.
These two types, or categories, of sleep are radically different from one another. It is a common misconception that when we are asleep our minds and bodies are simply ‘turned off’, and when we are awake they are ‘turned on’. In reality sleep is not so passive as you may think. Our bodies, and especially our minds are busy during sleep, and they are busy doing things that they only do while asleep – things that are necessary for health and growth.
In this classification of sleep we begin, believe it or not, with being awake. Scientists describe a stage of pre-sleep called Alpha, and include it within the category of quiet sleep. Normal sleep is numbered, stages 1 through 4, so I like to think of Alpha as stage 0 sleep.
Alpha begins the moment our eyelids shut. All visual sensory inputs are brought to a halt. We are in a state of calm, relaxed wakefulness. By wakefulness I don’t be we are wide awake and alert, just that we are awake. Once the eyes close our minds immediately begin to produce more ‘alpha waves’ (hence the name of the stage). These alpha waves correlate with increased relaxation, drowsiness and eventual progression to stage 1 sleep.
Stage 1 is best described as light sleep, or dozing. When you first fall asleep you will only spend about five minutes in this stage. This duration will increase in subsequent trips through the sleep cycle. You enter this stage when your brain begins to produce ‘theta waves’, which are much slower in frequency (4-7 cycles per second) and greater in amplitude than the alpha waves. As you begin to drift off into sleep you may notice that your mind begins to wander and also to ‘lapse’ from consciousness, and you probably experience powerful drowsiness. When this happens you are feeling the effects of the theta wave cycle in your brain.
During stage 1 sleep the body temperature begins to drop, the muscles throughout your body relax and the eyes move slowly from side to side. Although you begin to lose awareness of your surroundings it is easy to be startled back into wakefulness when in stage 1.
This is the first stage of established sleep. You will spend anywhere from ten to twenty five minutes in this stage during the first sleep cycle; longer in subsequent cycles. The brain continues to produce theta waves, but with an interesting twist. About once a minute your brain will experience a burst of fast activity that lasts about 1/2 second. These are called sleep spindles.
Another interesting phenomena is that every couple minutes another pattern called K-complex waves are produced. In these K-complex waves the amplitude of the waves increases dramatically. During these sleep spindles and K complexes it is much easier to be awakened. Some scientists believe this is a defense mechanism.
Test subjects under surveillance can be induced to produce K-complex patterns if someone whispers their name during stage 2 sleep. They can also be provoked by other stimuli, both external and internal, such as snoring, a loud noise or a stomach ache.
Also, during stage 2 sleep the eyes are still, and heart rate and breathing slow to below normal waking rates.
Stages 3 and 4 are known as deep sleep, or slow-wave sleep. They are characterized by slow and regular breathing, a drop in blood pressure and heart rate, and less responsiveness to stimuli. It is much more difficult to awaken someone who is in deep sleep. These stages are also referred to by scientists as ‘Delta sleep’ because during these stages the brain emits delta waves.
Stages 3 and 4 are the portion of sleep where the body is at work to renew and repair itself. A growth hormone is released in these stages that promotes tissue growth and muscle repair. This is why, after missing a couple days of sleep, people often report feeling stiff and sore in their joints and muscles. The immune system also gets a boost during this time.
Deep sleep makes up about 20% of a night of sleep in up to half-hour stretches for young adults. Unfortunately, adults over 65 years of age get much less deep sleep.
After a period of sleep deprivation a person will pass very quickly through the first two stages of sleep and head almost right into deep sleep. They will also spend a greater portion of their night in deep sleep. This indicates the importance of the later stages of sleep for restoration, and the body’s ability to adapt and recover from poor sleep.
Rapid Eye Movement (REM)
The last stage of sleep is in a category of its own, “Dreaming Sleep”. It is distinct from the other stages of sleep in a number of ways. REM sleep has been described as “An active brain in a paralyzed body”. Although we do dream in other stages of sleep, it is during dreaming sleep that our dreams are the most vivid and specific.
Unlike “Quiet Sleep”, in REM sleep the mind produces brain waves that are virtually the same as when we are awake. In fact, the brain races at a fast pace, extremely active. Additionally, the eyes zig and zag rapidly behind their eyelids, hence the name “rapid eye movement”.
Also, the body temperature rises, blood pressure increases, and heart rate and breathing rate speed up to levels similar to when you are wide awake. What’s more, the “flight-or-flight” mechanism is twice as active as when we are awake. Yet, in spite of all this activity, all muscles not involved in breathing or eye-movements become paralyzed, at least temporarily.
Whereas the deep sleep stages are primarily responsible for restoring our physical bodies, dream sleep restores our minds. Many scientists believe that our brains process the informational inputs of the day and clean out the useless information while associating and storing important information. A number of studies have shown that REM sleep facilitates learning and memory. When a person is repeatedly awakened out of dream sleep, for example, they lose much of the information they learned during the previous day.
REM sleep is critical for learning and memorization. This is why cramming all night for a test is not very helpful. You miss out on the primary way that the brain commits new information to long-term memory. On the other hand, information that you learn last, just before bed, seems to be what is processed by the mind first during REM sleep, so is most readily remembered the next day.
The first time through the sleep cycle REM sleep may only last a few minutes. However, in each subsequent cycle we spend progressively more time here, up to 30 minutes. As with deep sleep, if you get short sleep for a few days your REM sleep will rebound once you get a chance to have a good night of sleep.
The Sleep Cycle
During the night we pass through the different stages of sleep in about 90-100 minutes. This period of time constitutes a single sleep cycle, and most people pass through five or six cycles of sleep each night. As indicated, above, however, each cycle is not simply a repeat of previous cycles.
Most of the Stage 3-4 (deep) sleep occurs in the first half of the night. In the second half REM increases and alternates with stage 2 sleep. Stage 1 sleep is fairly short in any cycle, but may be skipped altogether in later cycles of the night.
Athletes anxious to get sleep the night before a competition can “rest assured” that if they only get a few hours of sleep they have received the majority of the physical restorative sleep.
Students will benefit from studying right before bed and then immediately going to sleep. Not only will the act of studying help make you sleepy, but you will get the best “bang for your buck”, so to speak because your brain will quickly process what was last studied during your early REM cycles.
Training yourself to wake up without an alarm may be easier than you think. This is because in the later cycles of sleep we spend more time in stage 2, during which we wake up more easily. Slight adjustments in the length of sleep, either to shorter or longer duration, may make it easy to wake up automatically as you complete a certain number of cycles and return to stage 2 sleep.