There are four stages of a sleep cycle, but for the sake of simplicity;
we'll treat them under two board categories; light sleep and deep sleep.
Depending on how active you are before you hit the bed, it takes
anywhere from five to thirty or more minutes for the average person to
get to the phase of light sleep. During the light sleep phase, you
respond more quickly to external stimuli such as sharp noises and bright
lights, toss and turn more reqularly, hence it is easier to wake up
from it.
After some time, you transition from the light sleep phase to the deep
sleep phase. During this phase your body and brain 'recharges' itself,
so to speak, from the day's activities. In a strange twist, your brain
is actually more active because of active dreaming. Still, you respond
slowly or not at all to external stimuli, and have little to no
movement.