RETRIEVAL PRACTICE Take a moment to answer each of these Learning Objective Questions (repeated here from within this section). Then click the 'show answer' button to check your answers. Research suggests that trying to answer these questions on your own will improve your long-term retention (McDaniel et al., 2009).

Sleep is the periodic, natural loss of consciousness—as distinct from unconsciousness resulting from a coma, general anesthesia, or hibernation. (Adapted from Dement, 1999.)

Our bodies have an internal biological clock, roughly synchronized with the 24-hour cycle of night and day. This circadian rhythm appears in our daily patterns of body temperature, arousal, sleeping, and waking. Age and experiences can alter these patterns, resetting our biological clock.

Younger adults cycle through four distinct sleep stages about every 90 minutes. (The sleep cycle repeats more frequently for older adults.) Leaving the alpha waves of the awake, relaxed stage, we descend into the irregular brain waves of non-REM stage 1 (NREM-1) sleep, often with hallucinations, such as the sensation of falling or floating. NREM-2 sleep (in which we spend the most time) follows, lasting about 20 minutes, with its characteristic sleep spindles. We then enter NREM-3 sleep, lasting about 30 minutes, with large, slow delta waves.
     About an hour after falling asleep, we begin periods of REM (rapid eye movement) sleep. Most dreaming occurs in this stage (also known as paradoxical sleep) of internal arousal but outward paralysis. During a normal night’s sleep, NREM-3 sleep shortens and REM and NREM-2 sleep lengthens.

Our biology—our circadian rhythm as well as our age and our body’s production of melatonin (influenced by the brain’s suprachiasmatic nucleus)—interacts with cultural expectations and individual behaviors to determine our sleeping and waking patterns.

Sleep may have played a protective role in human evolution by keeping people safe during potentially dangerous periods. Sleep also helps restore and repair damaged neurons. REM and NREM-2 sleep help strengthen neural connections that build enduring memories. Sleep promotes creative problem solving the next day. Finally, during deep sleep, the pituitary gland secretes a growth hormone necessary for muscle development.

Sleep deprivation causes fatigue and irritability, and it impairs concentration, productivity, and memory consolidation. It can also lead to depression, obesity, joint pain, a suppressed immune system, and slowed performance (with greater vulnerability to accidents).
     Sleep disorders include insomnia (recurring wakefulness); narcolepsy (sudden uncontrollable sleepiness or lapsing into REM sleep); sleep apnea (the stopping of breathing while asleep; associated with obesity, especially in men); night terrors (high arousal and the appearance of being terrified; NREM-3 disorder found mainly in children); sleepwalking (NREM-3 disorder also found mainly in children); and sleeptalking.

We usually dream of ordinary events and everyday experiences, most involving some anxiety or misfortune. Fewer than 10 percent of dreams among men (and less among women) have any sexual content. Most dreams occur during REM sleep.

There are five major views of the function of dreams. (1) Freud’s wish-fulfillment: Dreams provide a psychic “safety valve,” with manifest content (story line) acting as a censored version of latent content (underlying meaning that gratifies our unconscious wishes). (2) Information-processing: Dreams help us sort out the day’s events and consolidate them in memory. (3) Physiological function: Regular brain stimulation may help develop and preserve neural pathways in the brain. (4) Neural activation: The brain attempts to make sense of neural static by weaving it into a story line. (5) Cognitive development: Dreams reflect the dreamer’s level of development.
     Most sleep theorists agree that REM sleep and its associated dreams serve an important function, as shown by the REM rebound that occurs following REM deprivation in humans and other species.