5 Steps to a 5 AP Psychology, 2010-2011 Edition - Laura Lincoln Maitland [63]
Sleep and Dreams
Your finely tuned “biological clock,” controlled by the hypothalamus, systematically regulates changes in your body temperature, blood pressure, pulse, blood sugar levels, hormonal levels, and activity levels over the course of about a day. In an environment devoid of environmental cues to the length of a day, your free-running biological clock cycles approximately every 25 hours, but in a typical environment with light during the day and dark at night, cycles of changes, circadian rhythms, recur approximately every 24 hours. The forebrain, reticular formation, and thalamus are involved in the changes in wakefulness, arousal, and attention. Your physiological fluctuations are reflected in changes in your energy level, mood, performance, wakefulness, and sleep. Jet lag and night-shift work involve disruptions of circadian rhythms.
Why do you sleep? Evolutionary psychologists say that humans evolved a unique waking–sleeping cycle as a result of natural selection that maximized our chances of survival. Sleep serves at least two restorative functions—one involved in protein synthesis throughout the body, the other involved in maintaining plasticity of neural connections essential for storing and retrieving memories, which enables you to put together new material from the day before with old material. This is sometimes called consolidation. Sleep deprivation makes you drowsy, unable to concentrate, and impairs your memory and immune system. Sleep time seems to decrease from about 16 to 18 hours for a newborn, to about 7 to 8 hours for an adult.
Sleep is a complex combination of states of consciousness, each with its own level of consciousness, awareness, responsiveness, and physiological arousal. The amount we sleep changes as we age. Electroencephalograms (EEGs) can be recorded with electrodes on the surface of the skull. EEGs have revealed that brain waves change in form systematically throughout the sleep cycle (see Figure 9.1). When you are awake, your EEG shows beta waves when you are alert and alpha waves when you are relaxed. As you fall asleep, you pass into a semiwakeful state of dreamlike awareness, known as the hypnagogic state; you feel relaxed, fail to respond to outside stimuli, and begin stage 1 sleep. EEGs of stage 1 sleep show theta waves, which are higher in amplitude and lower in frequency than alpha waves. As you pass into stage 2, your EEG shows high-frequency bursts of brain activity (called sleep spindles) and K complexes. As you fall more deeply asleep, your stage 3 sleep EEG shows some very high amplitude and very low-frequency delta waves. In stage 4, your deepest sleep stage, EEGs show mostly delta waves. During stage 4, your heart rate, respiration, temperature, and blood flow to your brain are reduced. You secrete growth hormone involved in maintaining your physiological functions. Stages 1 through 4, during which rapid eye movements do not occur, are called NREM or Non-REM sleep. After passing through stages 1 through 4, you pass back through stages 3, 2, and 1; then, rather than awaking, you begin REM sleep (Rapid Eye Movement sleep) about 90 minutes after falling asleep. Your eyes jerk rapidly in various directions; your breathing becomes more rapid, irregular, and shallow; your heart rate increases; your blood pressure rises; and your limb muscles become temporarily paralyzed. Because your EEG shows beta activity typical of wakefulness and theta activity typical of stage 1 sleep, but you are truly asleep, REM sleep is often also called paradoxical sleep. Throughout the night, you cycle through the sleep stages with