HOW BODY RHYTHMS WORK: Circadian rhythms are biological cycles in the body that repeat approximately every 24 hours, including the sleep/wake cycle, along with body temperature, hormone levels, heart rate, blood pressure, and pain threshold. The brain has its own internal "pacemaker" that determines when nerve cells fire to set the body's rhythms, although scientists can't precisely explain how it does so. The colors of the light spectrum can affect the body's rhythm differently, particularly when it comes to sleep patterns. For instance, daylight is dominated by short, visible wavelengths of light that provides a blue visual sensation, like the blue sky. But how bright the light is, how far away, how long you're exposed and when you're exposed to light also have to be considered. Also, we are more likely to sleep soundly in the wee hours of the morning, when our body temperature is lowest, and most likely to awaken when our body temperature starts to rise, usually between 6 AM and 8 AM. As we age, the brain's "pacemaker" loses cells, changing circadian rhythms, especially sleep patterns. The elderly may nap more frequently, have disrupted sleep, or awaken earlier.
Stage 1: drowsiness
Stage 2: light sleep
Stages 3 and 4: deep sleep
Stage 5: Rapid-eye movement (REM) sleep.