Sleep apnea is characterized by pauses or lack of breathing when sleeping. A lack of oxygen to the brain causes a buildup of carbon dioxide in the bloodstream. Accumulated carbon dioxide causes the person to wake up, gasping for air. Normal breathing upon wakefulness will allow normal oxygen flow until the person falls back asleep. Sleep apnea is more common among middle aged and overweight individuals. While genetics play a significant role, those with enlarged tonsils, allergies (which may cause swelling of the throat), sinus problems, or enlarged necks are also at risk of sleep apnea.

Memory is encoded during REM sleep. In the hippocampus, neurons fire four times as opposed to once as a result of ponto-geniculo-occipital (PGO) spikes. Memories from a day's activities and learning are encoded in the hippocampus four times, contributing to long-term memory. Theta waves are also associated with this stage of sleep and contribute to REM's restorative nature.  

REM sleep (or Rapid Eye Movement) is the stage in which dreams occur, because even though the body is essentially paralyzed, this stage is accompanied by intense activity in the brain. 

The term "healthy lifestyle" may be broadly defined from culture to culture. While sleep is healthy and good for the body, too much sleep may also be detrimental, or a sign of severe depression. This answer choice is too broad, and not specific enough in its limitations on sleep, to be accurate.

All the other answers are well known theories about the function of sleep.

Out of the 4 stages of sleep, NREM-1, NREM-2, NREM-3, and REM, the third stage is where the brain will emit delta waves. These waves are large and slow, as such, this stage of sleep is also known as slow-wave sleep. It is during this stage where it may be difficult to wake someone up.

Dreaming has been observed during REM sleep. Brain waves at this stage resemble the brain waves of an individual who is awake. The person's heart rate will increase as well as his or her breathing. Every few seconds, the individual's eyes may be observed darting from side to side. This movement signifies the beginning of a dream.

While the recuperation explanation may seem like a good answer, it cannot be considered a theory on its own.

"Sleep protects" is a theory that a species will adjust its sleep pattern in a manner where it is optimal given that species niche, or environmental context. For instance, if an animal has a limited ability to hide from its predators, it may sleep less.

Sigmund Freud is well-known for his ideas of psychoanalysis—where an individual's personality is based on childhood memories. He is particularly famous for his concept of childhood progression through psychosexual stages. During the id phase, a child is consumed by his desires and need of instant gratification. Freud theorized that once a child graduated from the id stage, then he will achieve the ego stage and ultimately superego. At this time, the adult will have buried and overcome his desires in his unconscious and be able to pay attention to the conscious thoughts associated with morally and socially acceptable behaviors. Given this information, Freud would most likely be in support of dreams serving the purpose to satisfy one's wishes. He believed that dreams provided a platform to act out an individual's latent content—buried unconscious wishes and desires—in a censored symbolic version by way of manifest content. Manifest content is what the individual will remember as the dream's content and main storyline (i.e. what the dream was at "face value").

Dreaming has been observed to predominantly occur during rapid eye movement (REM) sleep. A dream may be noted at the beginning of REM sleep. If dreaming is to serve one function of reorganizing information into memory, then as some studies have indicated, uninterrupted REM sleep is most beneficial for integrating new information. Given that no stage 5 sleep has yet been identified, this would be an incorrect answer. Although some studies may counter-argue that interruptions in non-REM stages will yield poor memory integration, it is important to remember that a majority of dreaming does not occur in these stages.

Theta waves are usually connected to emotions, memories, and other activities in the limbic system. These cognitive processes calm when a person is transition from awake to stage 2 of sleep; therefore, the waves will be less frequent and higher in amplitude.