Sleep Cycles Vocabulary

Sleep and dreaming alter consciousness as part of the spontaneous state of consciousness.

The Circadian Rhythm

The circadian rhythm is our internal clock, controlling our temperature and wakefulness in 24-hour cycles. This rhythm lets us know when we feel tired and sleepy. Our thinking is sharpest, with memory being the most accurate when we are at our peak in circadian arousal.

You may not think so, but you are actually very familiar with your circadian rhythm. Remember jet lag? Whenever we travel to a place that is within a different timezone, we feel thrown off and unusually tired because jet lag disrupts the circadian rhythm. ✈️

The suprachiasmatic nucleus in the hypothalamus controls the circadian rhythm. In response to light, it causes the pineal gland to adjust melatonin production. In the morning, melatonin levels decrease, and in the evening, melatonin levels increase to get you prepared for sleep.

Why Sleep is Necessary

Evolutionary psychologists believe that sleeping became part of our behavior as a result of natural selection. In regard to AP Psychology, sleep is the periodic, natural loss of consciousness. The transition from a relaxed but awake state to sleep is marked by slower breathing and irregular brain waves.

Sleep serves various main functions: 

1. Sleep protects🛡️

2. Sleep helps us restore and repair brain tissue (maintaining plasticity) 🧠

3. Sleep restores and rebuilds our memories of the day 💭

4. Sleep feeds creative thinking 🎨

5. Sleep promotes growth (NREM-3) 🌱

Without sleep, we are unable to concentrate and often feel drowsy. These are all theories as to why we need sleep.

Breaking Down the Sleep Cycle

To measure sleep activity, neurologists use electroencephalograms (EEGs). Electrodes taped to the skull allow the EEG to produce an image of the electrical activity or waves in the brain:

Image Courtesy of Tuck

When awake and alert, the EEG shows beta waves. As you become more relaxed, alpha waves are shown.

Eventually, you fall into a dreamlike state, where you are semi-awake and feel relaxed, unable to respond to the environment or stimuli. This sleeping stage is called NREM-1, or the hypnagogic state. Theta waves are shown, and you may experience images resembling hallucinations, which may be incorporated into memories. A major example of this is when you wake up and think you’re falling; did that ever happen to you?

As sleep continues, you pass into NREM-2, where your EEG shows sleep spindles and K-complexes. Sleep spindles are sudden bursts of rapid brain wave activity. In the EEG attached, the sudden burst shown is a sleep spindle.

NREM-3, or deep sleep, follows NREM-2. During this sleep stage, delta waves are emitted, and growth hormones are released. Heart rate, respiration, and blood flow are reduced. The further into the night you get, the less deep sleep you have. 

Once you pass into REM sleep, vivid dreams occur, brain waves become rapid (beta waves), heart rate and breathing increase, and eye movements are rapid (hence REM [Rapid Eye Movement Sleep]). REM is also commonly labeled as paradoxical sleep, where muscles are relaxed while other body systems are active. As a result, waking up during REM sleep can cause sleep paralysis since you are awake but have limp muscles.

REM sleep is one of the most important topics to know for this section! Dreams and nightmares, in addition to relaxed muscles, are concepts commonly asked about on the AP Exam. The further into the night you get, the more REM sleep you have.

Summary of the Sleep Stages

Every 90 minutes, we cycle through the four sleep stages: 1-2-3-2-1-REM, then restart.

Image courtesy of Wikimedia Commons

Sleeping Disorders

While sleep is important to us as humans, sometimes problems can arise in regard to our sleep cycles. Insomnia, the inability to fall or stay asleep, can have detrimental health effects. On the other hand, narcolepsy, sudden uncontrollable sleep attacks, can have harmful effects on our ability to function on a day-to-day basis. 😴

Sleep apnea is another disorder that impacts our quality of sleep. People with sleep apnea randomly stop breathing while they are asleep and are frequently awakened throughout the night. 

Lastly, night terrors, which typically impact children, occur in NREM-3 and thus differ from regular dreams and nightmares. This may be characterized by incoherent chatter or physical movement. 👶

If you have ever been sleep-deprived, chances are you experienced REM rebound. The night after being sleep deprived, you spend more time in the REM sleep stage.

Dreaming

Sigmund Freud was interested in what dreams could tell us about our inner thoughts and desires. He believed that dreams had two messages. First was the manifest content, which was the actual remembered storyline. The second is referred to as the latent content or the underlying meaning of the dream. For example, being chased by an animal in a dream may actually mean we are worried about a deadline creeping up on us. 🐆

Freud’s theory as to why we dream is to satisfy our own wishes and deal with unconscious drives. Other theorists believe that we dream in order to file away memories, develop and preserve neural pathways, make sense of neural static, or reflect cognitive development. In reality, we are still learning a lot about both sleep and dreaming!

Vocabulary

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