It happens to most kids at one time or another: You wake up in the morning facing a new day -- only to yawn and wish you could go back to sleep.

Several things can cause you to feel tired, but there are things you can do to avoid it.

The amount of sleep you get each night is vital.

You may think eight hours sounds like enough, but it isn't. Experts say that kids ages 5 to 12 need about 10 to 11 hours of sleep every night.

Changes to your sleep schedule can also make you tired, and so can stress. Are you super-worried about a math test? That can affect how well you sleep. So can certain health issues. If your body is fighting a cold, it is going to require more sleep than when you are well.

Sleep may seem like a simple process, but it isn't. A lot of stuff is going on in your body and brain when you are slumbering.

There are five stages to what is called a sleep cycle. The entire cycle generally lasts 90 to 120 minutes and repeats about five times during the night. The first four stages account for about 75 percent of your night. During this period, your body goes from light to deep sleep.

Body functions such as blood pressure and body temperature drop. Muscles relax. The blood supply to your muscles increases and tissue growth and repair occurs. Hormones, such as those that help you grow, are released throughout your body. (You really do need to sleep to grow!)

Then in Stage 5, things change.

You go into what is called REM (rapid eye movement) sleep, meaning that your eyes dart back and forth. This is the time when your brain becomes active and most dreams occur. Your body becomes somewhat immobile.

Experts aren't sure why we dream, but they believe it is important in human development.

The National Sleep Foundation did a national survey a few years ago that found that many kids have sleep problems at least a few nights a week.