The average amount of sleep an adult gets is 8 hours. Everyone is different, however; where some adults function off of 6 hours, others need at least 9 hours or more to feel healthy and awake.

Circadian rythm is responsible for the cycles of our blood pressure, metabolism, and body temperature. It contributes to our levels of energy throughout the day and is responsible for our descension in sleepiness as night falls. 

After stage 1, which lasts about 5-10 minutes, is stage 2 during which we slowly enter deep sleep. This stage is still considered light sleep, corresponds to a drop in body temperature. We can still be woken up fairly easily. Stage 3 is when we enter heavier, deep sleep.

REM is arguably the most important stage of sleep, and it is the last stage of our sleep cycle until it repeats all over again. Due to the heightened activity in our brains, we may have more intense dreams during this period. Also during this time, our heart rate and breathing quickens, and our body restores itself.

Insomnia is a sleep disorder in which the affected person may not be able to fall asleep, and keeps waking up during the night. It affects a significant percentage of Americans, and is mostly caused by stress or some other underlying condition. It is often treated with medications or behavioral therapy.

A circadian rhythm is any process that abides by a 24-hour rhythm. It is best known for helping to tell our bodies when they need sleep. Having a disrupted circadian rhythm is related to negative health outcomes, including various neurological and cardiological problems.

Sleep occurs in 4 stages, NREM-1, NREM-2, NREM-3, and REM.

It's during the first stage of sleep (NREM-1) that you may experience images that resemble hallucinations. These experiences occur without a sensory stimulation and can sometimes be later incorporated into one's memories.

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.