For in that sleep of death what dreams may come
Sleep is one of the most mystical and yet truly quotidian events of life. Adult humans spend about a third of their lives sleeping. All homeothermic vertebrate mammals sleep. REM seems essential to restful sleep. My father, who lapsed into a deep depression near the end of his life, was afflicted with insomnia. His sleep, even the induced variety left him more tired when he woke. Perhaps he was unable to dream.
Sleep deprivation seems to be a favorite means of torture. Considered non-invasive, it still leaves scars on the psyche. Our circadian rhythms are disrupted, and this affects more than the body, as any medical student can affirm. Sleepless nights, whether in Seattle or not, can mean a terrible day. Polyphasic sleep, or short siestas can help ease the tiredness.
The physiological aspects of sleep are well studied, yet still little understood. The Non-REM and REM phases alternate, and the REM phase is the more erratic, causing changes in blood pressure, heart and breathing rate, as well as clitoral and penile engorgement. The brain is very active in REM, related to the dream states experienced.
A dream goes through the phases of hallucination, delusion, emotional identification, amnesia of sorts, and cognitive surrealism. REM has been found only in mammals and young birds, indicating that dreaming too is perhaps related to evolutionary changes. I've noticed my dog dreaming, and can tell why my young baby is having a dream. No way of telling though, what dogs and babies dream of.
The motivations for sleep lie in preserving circadian rhythms, although numerous theories point to metabolic, developmental, and learning/analysis explanations for sleep. A spiritualist might consider sleep a time for "dulcis sermonicinatio". Many a problem, challenge or tension is resolved in sleep.
A comfortable bed and sleeping position is paramount. This is a topic of much worth, and little agreement. I tend to adopt a semi-fetal position to my right, on an IKEA Malmo futon, for now. Despite all theories to the contrary, women own the bed, and we men are mere guests at their convenience. That too, is a topic of some interest, but perhaps for another time.