To sleep: perchance to dream: ay, there's the rub (Shakespeare, Hamlet, III, i).
What is it to sleep and perchance to dream? The real rub is located in the MedicineNet Medical Dictionary. The entry to "Dreaming" there follows:
Dreaming: Dreaming can be defined in various ways. For one, it is having thoughts, visions, and other sensations that occupy the mind in sleep.
Dreams occur during that part of sleep when there are rapid eye movements (REMs). We have 3 to 5 periods of REM sleep per night. They usually come at intervals of 1-2 hours and are quite variable in length. An episode of REM sleep may be brief and last but 5 minutes. Or it may be much longer and go for over an hour.
About 20% of sleep is REM sleep. If you sleep 7-8 hours a night, perhaps an hour and half of that time, 90 minutes, is REM sleep.
REM sleep is characterized by a number of other features besides REM, including rapid, low-voltage brain waves on the electroencephalographic (EEG) recording, irregular breathing and irregular heart rate and -- what may be most evident to someone else -- involuntary muscle jerks.
Non-REM (NREM) sleep is dreamless sleep. During NREM, the brain waves on the EEG are typically slow and of high voltage, the breathing and heart rate are both slow and regular, the blood pressure is low, and the sleeper is relatively still. NREM sleep is divided into 4 stages of increasing depth.
About 80% of sleep is NREM sleep. If you sleep 7-8 hours a night, all but maybe an hour and a half is spent in dreamless NREM sleep.
Dreams are penetrable; it has been found experimentally that one can communicate with a person who is dreaming.
Dreaming is not uniquely human. Cats and dogs dream, judging from the physiologic features. So apparently do many other animals.
The word "dream" is traditionally traced back to an Anglo-Saxon word meaning joy, gladness, or mirth. However, "dream" more likely came from another word (from Sanskrit) meaning deception. Thus, when we dream, is it a joy or a deception? (Let your subconscious be the judge!)
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