专四听写训练51：How and Why We Sleep?
It's apparent that everyone needs to sleep.However,most people rarely think about how and why they sleep.We know that if we sleep well,we feel rested.If we don't sleep enough,we often feel tired and irritable.It seems there are two purposes of sleep:physical rest and emotional and psychological rest.We need to rest our bodies and our minds. Both of which are important in order for us to be healthy.
Each night we alternate between two kinds of sleep:active sleep and passive sleep.The passive sleep gives our body the rest and prepares us for active sleep in which dreaming occurs. Throughout the night,people alternate between passive and active sleep.The brain rests then it becomes active then dreaming occurs.The cycle is repeated and this cycle is repeated several times throughout the night.During eight hours of sleep,people dream for a total of one and a half hours on the average.