By MIKE MENNELLA

All animals, including man, sleep. Sleep is characterized by the loss of consciousness and the slowing of the various functions of your mind and body. Sleep is necessary because it preserves your life. Your body also recuperates during sleep.

What causes sleep? There are many theories about the cause of sleep, but many physiologists believe that it's a combination of bodily changes.

Some of the most typical changes occurring in sleep have to do with the breathing and circulation. The rate of breathing is slower when you're asleep. Your heartbeat also is slowed down during sleep.

Other changes occurring in sleep include lowering of the body temperature and relaxation of your muscles. Although the muscles are relaxed during sleep, they are not always relaxed at the same time; and a certain amount of movement occurs. Studies have shown that the average

sleeper changes his position about once every 17 minutes during his sleeping period.

One part of your body that doesn't change while you're sleeping is your brain. That is one of the reasons people dream while they're asleep.

How much sleep should the average person get? The amount of sleep you should get depends on your age. The older you are the less sleep you need. The average amount of sleep is 11 1/4 hours at six years, 10 hours at 10 years, nine hours at 15 years, 8 1/4 hours at 20 years, eight hours at 25 years, and seven hours at 69 years of age.

Coffee with caffeine is said to keep people awake. Experiments have shown that this is not true. Most people are not affected by coffee. People who have trouble sleeping or walk in their sleep are found to have psychological difficulties. There is one thing you can be sure of, though; everybody sleeps at one time or another.