Contrary to what is often thought, sleep is actually an active, organized process. How and when we sleep is governed by a number of factors. These include factors under our control, such as whether or not we are sleep deprived, and factors beyond our control. Chief among these is our internal biologic clock that regulates our biologic rhythm (also called a circadian rhythm) over a 24-hour period. Sleep also has an internal organization regulated by different areas of the brain.
Sleep actually occurs in stages, which occur at different times during the night. There are two major divisions of our sleep state. These are called rapid eye movement (REM) sleep, and non-rapid eye movement (Non-REM) sleep. Non-REM sleep is in turn further divided into four different stages (1 through 4), with stages 3 and 4 often referred to as "deep sleep." In adults, non-REM sleep occupies around 80 percent of the night, and REM sleep 20 percent. However, REM sleep does not occur in one large block. Actually, we go into REM sleep in cycles of around 90 minutes. That is, REM sleep occurs around once every 90 minutes.
During non-REM sleep, many of the restorative functions of sleep occur. Hormones are released which help the body rebuild itself from damage done during the day. During REM sleep, memories and thoughts from the day are processed. REM sleep is the stage of sleep in which vivid dreams occur. The purpose of dreaming is not well understood, but it probably relates to processing mental information that was received during the day. During REM sleep, we normally lose the use of our limb muscles. Thus, we have an active mind in an inactive body. This normal loss of muscle activity in REM (or dream) sleep helps prevent us from acting out our dreams.
Different sleep disorders may occur during different stages of sleep. For example, sleepwalking and night terrors, common problems in children, usually occur in non-REM sleep. There are disorders of REM sleep in which the normal loss of muscle tone is absent. Affected patients may act out violent dreams and harm themselves or others.
There is, in fact, a wide range of sleep time that is considered "normal." While the average normal amount of sleep is around 7.5 hours per night, there are some people who do just fine on 5 hours per night, and some who require as much as 9 hours per night. The key is to find the right amount for you. The best way to tell is by seeing how you function during the day.
For example, if after 6 hours of sleep you feel refreshed in the morning and awake during your daylight hours, then you don't need more than that. If, on the other hand, you need 9 hours a night to feel refreshed and to function well during the day, then that is what your individual requirement is and sleeping the "normal" amount of 7.5 hours per night will actually leave you sleep deprived.
If you are getting what you consider to be an adequate amount of sleep and are still unrefeshed and sleepy, then you might have an organic sleep disorder and should consider seeking professional consultation.