13-1 A Clock for All Seasons

Biorhythms are cyclic behavior patterns of varying length displayed by animals, plants, even single-celled organisms. Biorhythms displayed by mammals include, among others, circadian (daily) rhythms and circannual (yearly) rhythms. In the absence of environmental cues circadian rhythms are free-running, lasting a little more or a little less than their usual period of about 24 hours depending on the individual organism or the environmental conditions. Cues called Zeitgebers reset biological clocks to a 24-hour rhythm. Circadian rhythms allow us to synchronize our behavior with our body’s metabolic processes—so that we are hungry, and at optimal times. Environmental intrusions into our natural circadian rhythm, from artificial lighting to jet lag, contribute to metabolic syndrome. Biological clocks produce epigenetic effects: they regulate gene expression in every cell in the body.

13-2 Neural Basis of the Biological Clock

A biological clock is a neural structure responsible for producing rhythmic behavior. Our master biological clock is the suprachiasmatic nucleus. The SCN is responsible for circadian rhythms; it has its own free-running rhythm with a period of a little more or a little less than 24 hours. Stimuli from the environment, such as sunrise and sunset, meals, or exercise, entrain the free-running rhythm so that its period approximates 24 hours.

SCN neurons are active in the daytime and inactive at night. These neurons retain their rhythmicity when disconnected from other brain structures, when removed from the brain and cultured in a dish, and after culture in a dish for many generations. When reimplanted in a brain without an SCN, they restore the animal’s circadian rhythms. Aspects of neuronal circadian rhythms, including their period, are under genetic and epigenetic control.

13-3 Sleep Stages and Dreaming

Sleep events are measured by recording the brain’s activity to produce an electroencephalogram (EEG), muscular activity to produce an electromyogram (EMG), and eye movements to produce an electrooculogram (EOG).

A typical night’s sleep, as indicated by physiological measures, consists of stages that take place in cycles over the course of the night. During REM sleep the EEG displays a waking pattern and the sleeper displays rapid eye movements. Sleep stages in which the EEG has a slower rhythm are called non-REM (NREM) sleep.

Intervals of NREM sleep and REM sleep alternate four or five times each night. The duration of NREM sleep periods is longer earlier in sleep, whereas the duration of REM sleep periods is longer in the later part of sleep. These intervals also vary with age.

A sleeper in NREM has muscle tone, may toss and turn, and has dreams that are not especially vivid. A sleeper in REM sleep has vivid dreams in real time but has no muscle tone and so is paralyzed. Dream duration coincides with the duration of the REM period.

The activation–synthesis hypothesis proposes that dreams are not meaningful, merely a by-product of the brain’s state of excitation during REM. The coping hypothesis suggests that dreaming evolved as a mechanism to deal with challenges and fears posed by life.

13-4 What Does Sleep Accomplish?

Several theories of sleep have been advanced, but the main proposition is that sleep is a biological adaptation that conserves energy. Sleep is suggested as a restorative process that fixes wear and tear in the brain and body. Sleep also organizes and stores memories.

13-5 Neural Bases of Sleep

Separate neural regions are responsible for NREM and REM sleep. The reticular activating system, located in the central brainstem, is responsible for NREM sleep. If the RAS is stimulated, a sleeper awakes; if it is damaged, a person may enter a coma.

The peribrachial area and the medial pontine reticular formation in the brainstem are responsible for REM sleep. If these areas are damaged, REM sleep may no longer occur. Pathways projecting from these areas to the cortex produce the cortical activation of REM, and those projecting to the brainstem produce the muscular paralysis of REM.

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13-6 Sleep Disorders

Disorders of NREM sleep include insomnia, the inability to sleep at night, and narcolepsy, inconveniently falling asleep in the daytime. Sedative-hypnotics used to induce sleep may induce drug dependence insomnia, a sleep disorder in which progressively larger doses are required to produce sleep.

Disorders of REM sleep include sleep paralysis, in which a person awakens but remains unable to move and sometimes feels fear and dread. In cataplexy, caused by a loss of orexin cells in the brain, a person collapses into a state of paralysis while awake. At the same time the person may have hypnogogic hallucinations similar to dreaming. In REM sleep behavioral disorder a sleeping person acts out dreams.

13-7 What Does Sleep Tell Us about Consciousness?

Sleep research provides insight into consciousness by revealing many kinds of waking and sleeping. Just as the events of wakefulness intrude into sleep, the events of sleep can intrude into wakefulness. The array of conditions thus produced demonstrates that consciousness is not a unitary state.