Another mystery about sleep is that no two persons need the same amount of sleep. Dr. Nathaniel Kleitman, Associate Professor of Physiology at the University of Chicago, who conducted years of extensive experiments at the University's "Sleeping Laboratory" says that there is no more a normal duration of sleep than there is normal height and weight. A study of 25 subjects spread over thousands of nights showed that the average amount of sleep needed to feel well rested is seven-and-a-half hours, though individuals varied from six to nine hours. According to Dr. Demmis Williams, a noted authority on sleep, the amount of sleep needed for an individual's well-being, is determined by what he feels he needs, not by what other people, including the doctor, think is reasonable.

On the whole, women sleep from 45 minutes to one hour more than men. The amount of sleep required varies at different ages as follows :

New Born : 18 to 20 hours
Growing children : 10 to 12 hours
Adults : 6 to 9 hours
Aged persons : 5 to 7 hours

The depth of ordinary restful sleep fluctuates throughout the sleep. In most adults, sleep deepens through the first hour, after which it lightens rather sharply and then more gradually until morning or until the usual time of wakening. IN growing children, however , sleep deepens a second time for a little while. According to Dr. Lindlahr, a famous naturopath, two hours before and two hours after midnight are the most valuable for sleep of all the twenty-four hours of the day. In these four hours, mental and physical vigour are at their lowest ebb and sleep is soundest and most natural.

It is believed that three-quarters of our sleep consists of whatis called ' slow wave sleep.' The restorative processes occur during this time. The remaining quarter is taken by what is called 'rapid eye movement (REM) sleep.' It is also called paradosical or dreaming sleep and it comes in episodes of about 20 minutes duration about five times in a night. It involves dreaming, irregular heart rates, raised blood pressure and erection of the penis. It is in this phase of sleep that normal healthy young men may have wet dreams. Both forms of sleep are considered equally important, being normal sleeping rhythms.