It is safe for us to assume that for as long as there has been life on Earth, people have applied considerable thought to why we sleep and dream. Ancient cultures nurtured many weird and wonderful ideas with regard to sleep. The inhabitants of classical Greece were of the opinion that Hypnos, god of sleep, watched over us as we rested and dreamed. Another view, one which prevailed up until the time of the Renaissance, pertained to the flow of life-giving juices, the idea being that this flow was turned off when the body was sleeping and turned on again upon waking. Even today we are unable to fully explain the significance of sleep and dreaming, and its physiological benefits. What we are sure of, however, is that both humans and animals need sleep in order to survive.
Modern day research into sleep and the conscious mind is highly reliant on so-called polygraphic technology. This involves measuring and registering a series of bodily functions, such as eye movements, muscle activity, pulse frequency, blood pressure, etc.
When we sleep, both body movements and energy usage decrease, and our body temperature drops by about 0.5 degrees Celsius. Our body's metabolism also slows down, and activity in the brain is reduced by 15-20%. Activity in the frontal lobes of the brain especially is reduced, an area that has a direct influence on our thoughts and actions when awake. This can be assumed to indicate a state of relaxation for the brain.
So-called sleep-jerks are a common phenomenon, occurring just after we fall asleep. These are quite normal, and stem from the brain relaxing its grip on muscle tension when the body subsides into a state of sleep. Should this occur abruptly and with force, however, a person can be woken up equally abruptly, and experience a sensation of falling.
When we sleep, our blood pressure and pulse rate is reduced. Blood vessels dilate and the flow of blood increases, causing our skin to become warm and red. Our breathing rate also slows down.
Researchers have established a definite link between what happens in our dreams, and changes in blood circulation, breathing and eye movements.