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.
The three stages of quiet (non-REM) sleep. Stage N1. In making the transition from wakefulness into light sleep, you spend about five minutes in stage N1 sleep. On the EEG, the predominant brain waves slow to four to seven cycles per second, a pattern called theta waves (see Figure 1).