Melatonin is a hormone known to regulate sleep and wake cycles. It is released by the pineal gland into the bloodstream when sleep is prompted.

The pineal gland is in communication with the SCN. When the SCN, a structure in the hypothalamus, senses light from the retinas, it will suppress the release of melatonin from the pineal gland. Thus, we're usually awake during the day time and asleep at night.

Insomnia is a disorder that is characterized by an abnormal difficulty to fall or stay asleep. There are more than three million cases in the United States each year—roughly one percent of the population—and its prevalence is most common among older individuals.

From stage 1, stage 2, stage 3, stage 4, and REM sleep, the average human sleeps about 90-110 minutes in a typical cycle.

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.