There are some physiological changes associated with sleep changes. This is muscle sympathetic nerve activity reported in the perineal nerve. This is blood pressure and as you can see, as the subject goes from awake to non-REM sleep the sympathetic activity declines progressively. Blood pressure declines and then in REM you can see this enormous increase in sympathetic activity with wide swings in blood pressure. This is in a normal subject sleep apnea, narcolepsy, insomnia, daytime sleepiness.
Sleep is a very organized behavior.
There is a specific pattern of sleep across the night. That episode is brief. Sometimes as short as five minutes then you go back into non-REM, have some more delta sleep and Insufficient sleep. Individual sleep need varies. There is no right amount of sleep to get although most people in our society are sleep deprived and should you evaluate the patients with complaints of daytime sleepiness you have to factor that in.