Analysis of enucleated eyes has demonstrated histological features characteristic of nevi within the edges of malignant melanomas, suggesting that at least some melanomas may evolve from malignant transformation within pre-existing nevi.8 Nevertheless, the risk that a choroidal nevus may turn into a melanoma is generally considered extremely low.
Stages of Sleep. During sleep, we usually pass through five phases of sleep: stages 1, 2, 3, 4, and REM (rapid eye movement) sleep. These stages progress in a cycle from stage 1 to REM sleep, then the cycle starts over again with stage 1. We spend almost 50 percent of our total sleep time in stage 2 sleep, about 20 percent in REM sleep, and the remaining 30 percent in the other stages. Infants, by contrast, spend about half of their sleep time in REM sleep. During stage 1, which is light sleep, we drift in and out of sleep and can be awakened easily. Our eyes move very slowly and muscle activity slows. People awakened from stage 1 sleep often remember fragmented visual images.
Polysomnography monitors your sleep stages and cycles to identify if or when your sleep patterns are disrupted and why. The normal process of falling asleep begins with a sleep stage called non-rapid eye movement (NREM) sleep. During this stage, your brain waves, as recorded by electroencephalography (EEG), slow down considerably. Your eyes don't move back and forth rapidly during NREM, in contrast to later stages of sleep. After an hour or two of NREM sleep, your brain activity picks up again, and rapid eye movement (REM) sleep begins. Most dreaming occurs during REM sleep. You normally go through four to six sleep cycles a night, cycling between NREM and REM sleep in about 90 minutes.