Many factors play a role in preparing your body to fall asleep and wake up. You have an internal "body clock" that controls when you're awake and when your body is ready for sleep.
The body clock typically has a 24-hour repeating rhythm (called the circadian rhythm). Two processes interact to control this rhythm. The first is a pressure to sleep that builds with every hour that you're awake. This drive for sleep reaches a peak in the evening, when most people fall asleep.
A compound called adenosine (ah-DEN-o-seen) seems to be one factor linked to this drive for sleep. While you're awake, the level of adenosine in your brain continues to rise. The increasing level of this compound signals a shift toward sleep. While you sleep, your body breaks down adenosine.
A second process involves your internal body clock. This clock is in sync with certain cues in the environment. Light, darkness, and other cues help determine when you feel awake and when you feel drowsy.
For example, light signals received through your eyes tell a special area in your brain that it is daytime. This area of your brain helps align your body clock with periods of the day and night.
Your body releases chemicals in a daily rhythm, which your body clock controls. When it gets dark, your body releases a hormone called melatonin (mel-ah-TONE-in). Melatonin signals your body that it's time to prepare for sleep, and it helps you feel drowsy.
The amount of melatonin in your bloodstream peaks as the evening wears on. Researchers believe this peak is an important part of preparing your body for sleep.
Exposure to bright artificial light in the late evening can disrupt this process, making it hard to fall asleep. Examples of bright artificial light include the light from a TV screen, computer screen, or a very bright alarm clock.
As the sun rises, your body releases cortisol (KOR-tih-sol). This hormone naturally prepares your body to wake up.
The rhythm and timing of the body clock change with age. Teens fall asleep later at night than younger children and adults. One reason for this is because melatonin is released and peaks later in the 24-hour cycle for teens. As a result, it's natural for many teens to prefer later bedtimes at night and sleep later in the morning than adults.
People also need more sleep early in life, when they're growing and developing. For example, newborns may sleep more than 16 hours a day, and preschool-aged children need to take naps.
Young children tend to sleep more in the early evening. Teens tend to sleep more in the morning. Also, older adults tend to go to bed earlier and wake up earlier.
The patterns and types of sleep also change as people mature. For example, newborn infants spend more time in REM sleep. The amount of slow-wave sleep (a stage of non-REM sleep) peaks in early childhood and then drops sharply after puberty. It continues to decline as people age.
For more information about what makes you sleep, go to the National Heart, Lung, and Blood Institute's "Your Guide to Healthy Sleep."