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Dreams occur mainly during the phase of REM sleep in which brain activity is more intense. Dreams can also occur during other phases of sleep. However, these dreams tend to be much less vivid and memorable.
The length of a dream can vary, in fact, it can last from a few seconds to about 20-30 minutes. People are more likely to remember their dreams if they are awakened during the REM sleep phase....

This sleep disorder called exploding head Syndrome is a very rare form of auditory hallucination.
It is a parasomnia in which subjects experience a loud bang in their head similar to the explosion of a bomb, a cymbal clash, the ringing of a bell or an earthquake. The sound is garbled and seems to come from inside the head....

Dolphins are capable of what is known as the Unihemispheric sleep, in which only a brain hemisphere goes into a deep sleep while the other remains awake for the control of vital functions like, in the first place, respiration, since this act is voluntary...

During the first 90 minutes of sleep, you move from light sleep to deep sleep. Then, you enter Rapid eye movement sleep (REM); the stage during which dreams occur.
During this phase, the motor neurons and the release of certain neurotransmitters, the monoamines (norepinephrine, serotonin and histamine) are inhibited, which causes muscle weakness, rapid eye movements, irregular heartbeat and almost a total paralysis...