A nightmare is a dream occurring during REM sleep characterized by feelings of anxiety, fear, distress, sadness, guilt, or terror. Nightmares are often confused with night terrors. These two types of disturbing dreams can be differentiated both biologically and psychologically. <br/><br/>Unlike nightmares, which usually wake the dreamer and are often clearly recalled upon waking, night terrors typically don't wake the dreamer and are most often not remembered in the morning. One who suffers a night terror usually experiences confusion and a pervasive feeling of fear or anxiety upon waking, but is not often able to recollect the substance of the episode or the vivid imagery typically associated with nightmares. <br/><br/>Unlike nightmares, which occur generally during REM stages in the second half of the night, night terrors occur during slow-wave non-REM periods, usually within the first few hours of sleep. <br/><br/>The passage highlights the ways in which night terrors differ from nightmares, except for the following: