They are divided into 2 major parts:
First, non-REM (rapid eye mouvement) session, divided in 4 stages, and accounts 80% of the sleeping cycle:
- Stage 1: Sort of a gateway state between wake and sleep aka ''somnolence'' or ''drowsy sleep''. It's also a transition state into stage 2,
- Stage 2: Cconscious awareness of the external environment disappears. This occupies 45–55% of the cycle.
- Stage 3: Transition stage into stage for, occupies 3 to 8% of the sleep cycle.
- Stage 4: This is the stage in which night terrors, bed wetting, sleepwalking, and sleep talking occur. The non-REM session is also characterized by parasympathetic system conserves energy as it slows the heart rate, increases intestinal and gland activity.
Second, REM session (about 20% of the cycle):
REM sleep is characterized by loss of skeletal muscle tone, and sympathetic nervous system activity (it prepares the body to awakeness).
Source: wikipedia (search for sleep)
In order to rest fully, you should have several complete sleep cycles, instead of sleeping little by little. For instance, sleeping 5 hours will make you wake up in a middle of a cycle, and you will feel even more tired than if you slep 4.5hours straight (3 sleep cycles). And sleeping 20 minutes every hour will be completely useless, even if you total hours of sleep at the end (except if you're a polyphasic sleeper of course).
Here are 2 studies I found on Glen Rhodes' site:
"A group of Harvard scientists trained volunteers to perform a visual task that required them to learn how to recognize certain patterns as they flashed quickly on the computer screen. When the subjects were tested 10 hours later, those who had taken a 90-minute nap did much better than those who didn't nap. In fact, they did as well as people who got a full night's sleep in a previous study" - http://www.sleepfoundation.org/Alert/030730.cfm
Here's something from the Center for Applied Cognitive Studies (http://centacs.com)
"Studies show that the length of sleep is not what causes us to be refreshed upon waking. The key factor is the number of complete sleep cycles we enjoy. Each sleep cycle contains five distinct phases, which exhibit different brain- wave patterns. For our purposes, it suffices to say that one sleep cycle lasts an average of 90 minutes: 65 minutes of normal, or non-REM (rapid eye movement), sleep; 20 minutes of REM sleep (in which we dream); and a final 5 minutes of non-REM sleep. The REM sleep phases are shorter during earlier cycles (less than 20 minutes) and longer during later ones (more than 20 minutes). If we were to sleep completely naturally, with no alarm clocks or other sleep disturbances, we would wake up, on the average, after a multiple of 90 minutes--for example, after 4 1/2 hours, 6 hours, 7 1/2 hours, or 9 hours, but not after 7 or 8 hours, which are not multiples of 90 minutes. In the period between cycles we are not actually sleeping: it is a sort of twilight zone from which, if we are not disturbed (by light, cold, a full bladder, noise), we move into another 90-minute cycle. A person who sleeps only four cycles (6 hours) will feel more rested than someone who has slept for 8 to 10 hours but who has not been allowed to complete any one cycle because of being awakened before it was completed.... "
"Studies show that the length of sleep is not what causes us to be refreshed upon waking. The key factor is the number of complete sleep cycles we enjoy. Each sleep cycle contains five distinct phases, which exhibit different brain- wave patterns. For our purposes, it suffices to say that one sleep cycle lasts an average of 90 minutes: 65 minutes of normal, or non-REM (rapid eye movement), sleep; 20 minutes of REM sleep (in which we dream); and a final 5 minutes of non-REM sleep. The REM sleep phases are shorter during earlier cycles (less than 20 minutes) and longer during later ones (more than 20 minutes). If we were to sleep completely naturally, with no alarm clocks or other sleep disturbances, we would wake up, on the average, after a multiple of 90 minutes--for example, after 4 1/2 hours, 6 hours, 7 1/2 hours, or 9 hours, but not after 7 or 8 hours, which are not multiples of 90 minutes. In the period between cycles we are not actually sleeping: it is a sort of twilight zone from which, if we are not disturbed (by light, cold, a full bladder, noise), we move into another 90-minute cycle. A person who sleeps only four cycles (6 hours) will feel more rested than someone who has slept for 8 to 10 hours but who has not been allowed to complete any one cycle because of being awakened before it was completed.... "
Cactus
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