Obesity and depression are often associated with sleep apnea. This comorbidity is not commonly observed with the other choices: night terrors and ADHD.

Parasomnias are disorders that occur during sleep. There are various types of parasomniaas; however, nightmare disorder most commonly occurs during REM—rapid eye movement—sleep stage.

Light sleep, memory consolidation, and synaptic pruning characterize stage 2 sleep. This stage has higher-voltage sleep spindles and K-complexes. It is the transition period from light sleep (stage 1) into slightly deeper sleep (stage 3) and subdued muscle movement.

People typically sleep in five stages: one, two, three, four, and REM. Sleep begins in stage one and progresses to REM. People often cycle back to stage one and then sleep through another cycle. Each cycle typically lasts between 90 and 110 minutes. People dream during REM sleep but are normally paralyzed during this stage, which becomes longer as the night goes on. It is during stage four that sleepwalking typically occurs.

Delta waves are the slowest waves and occur during periods of deep sleep. It is also known as slow wave sleep. The answer would not be REM sleep, because it is characterized by saw-tooth waves. Sleep onset stage may be characterized by alpha (i.e. drowsy) and theta (i.e. light sleep) waves. Stage two of sleep typically shows k-complexes and sleep spindles, which are not seen with delta waves. 

REM (rapid eye-movement) sleep is the fifth and final stage of sleep during which the body is in it's deepest sleep. Dreaming is most frequent, most vivid, and most memorable during REM sleep; however, other dreaming can occur during the other sleep stages. 

The circadian rhythm is the 24-hour biological cycles found in humans and other species. Circadian rhythms are influential in regulating sleep, blood pressure, urine production, hormonal secretions, and other physical functions. Although each of these could be affected, the most comprehensive answer is circadian rhythm. 

Over the course of the four repetitions, REM sleep starts as a fairly brief period of about 45 minutes. By the end of the fourth repetition, the REM period of the sleep cycle is substantially longer. Dreaming is most associated with REM sleep, and dreaming during REM stays the same over the course of the sleep cycle. 

Sleep deprivation and partial sleep deprivation have many negative consequences on our physical and mental well-being. Studies have shown that people who consistently sleep less than 7 hours are at increased risk for overall mortality. 

Sleep can be divided into REM and NREM sleep. Stages 1 to 4 are characterized by non-rapid eye movement (i.e. NREM) sleep. Stage 5 is characterized by rapid eye movement (i.e. REM). Beta waves occur during normal wakefulness and are not associated with sleep. As the body begins to fall asleep and enters stage 1, alpha waves become present. During stage 2 theta waves and sleep spindles emerge. During stages 3 and 4, delta waves are present.