We all look forward to a good night's sleep. Sleep allows our body to rest and to restore its energy levels. Without enough restful sleep, not only can we become grumpy and irritable, but also inattentive and more prone to accidents. Like food and water, adequate sleep is essential to good health and quality of life.
There are two types of sleep: non-rapid eye movement -- or NREM sleep -- and rapid eye movement -- or REM sleep. NREM sleep includes four stages, ranging from light to deep sleep. We cycle through these four stages of sleep approximately every 90 minutes. Then we go into REM sleep, the most active stage of sleep when dreaming often occurs. During REM sleep, the eyes move back and forth beneath the eyelids and muscles become immobile.
Researchers believe that two body systems -- the sleep-wake process and our circadian biologic clock -- regulate our sleep. They program our bodies to feel sleepy at night and awake during the day.
The sleep-wake process works by balancing the amount of sleep a person needs based on the time spent awake. Our circadian biologic clock is a 24-hour body rhythm affected by sunlight. It regulates hormones such as melatonin, which is secreted during the night and promotes sleep, and other processes like body temperature. Sleeping at a time that is in sync with this rhythm is important for healthy sleep.