There are different stages of sleep. Wake, I, II, Delta, and REM. Technicians and doctors often use the electrical signals from your brain and the way your body reacts to tell what stage of sleep you are in. This is a small insight to the stages.
Wake- This is when you are alert and active. You are now tired, or dozing off.
Stage I of sleep- In stage I, your eyes begin to lightly roll, and your brain begins to slow down. This is such a light stage of sleep that many people will not even know that they were sleeping. This stage does not last very long for most people who have uninterrupted sleep.
Stage II of sleep- In stage II, this is a stage when you know you are asleep, but can be woken up fairly easily. Your brain slows down a little more, and your body begins to relax more.
Delta- This is the deepest stage of sleep. Many men as they get older may not even get into this stage of sleep. It is much more difficult to wake someone up from this stage of sleep. Delta is a combination of stage III and IV, which used to be seperate stages of sleep. This is a stage of sleep which promotes physical healing.
REM- This is what people refer to as dream stage of sleep. This is the stage where you dream, and was once thought of a time for your body to heal itself emotionally. This stage can also be the most dangerous stage for people with hypoxia and sleep apnea.
As you age through life, the amount of each type of sleep you get changes. When you are a baby, REM is very prominant. As a kid, delta is increased. This explains why some kids can sleep through almost everything. The amount of delta gradually decreases as an adult, and in some men can disappear completely.
How you are able to cycle through sleep can contribute to how some people can sleep for 10 hours and still be tired, while others do just fine on 6.