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.