Overview (P. 497)

• Sleep is defined behaviorally by the suspension of normal consciousness and electrophysiologically by specific brain wave criteria.
• Sleep consumes fully a third of our lives - what really happens to our body while we sleep?
• One thing for sure is, sleep is not the result of simple diminution of brain activity; rather, sleep is a series of precisely controlled brain states.
• To understand this concept, let us follow the brain activity, by electroencephalogram (EEG; Box A) in a normal human (Fig. 26.1).
• The waking state is characterized by high-frequency, low amplitude activity. These waves are called beta (eyes open) and alpha (eyes closed) waves.
• This state is followed by non-rapid eye movement (non-REM) sleep; which in itself has 4 stages.
• Non-REM sleep is characterized by decreasing frequency and increasing amplitude of EEG waves. These waves are called slow-waves or delta waves.
• Following non-REM sleep, the person's EEG goes back to low-voltage, high-frequency activity that is remarkably similar to the EEG activity of individuals who are awake. This stage is called REM sleep.
• A typical night's sleep is shown in Fig. 26.2. In total then, the typical 8 hours of sleep is divided into about 1.5 to 2 hours of REM sleep and about 6 hours of non-REM sleep.

Non-REM Sleep (P. 497)

• There are several evidence that suggest that non-REM sleep is restorative in function.
• First, the metabolism of the brain during slow-wave sleep, as measured by cerebral blood flow, is reduced by as much as 45% (Fig. 26.3).
• Second, non-REM sleep deprivation in lab animals eventually lead to a breakdown of homeostatic function, and ultimately to death.
• Despite the fact that the brain is relatively quiescent during the non-REM sleep, the body is remarkably active. This is the period when sleepwalking commonly occurs.

REM Sleep (P. 499)

• During REM sleep the eyes move rapidly; hence the name rapid eye movement sleep.
• The body does not respond to stimuli during this stage, despite the fact that the brain is active.
• The REM sleep waves originate in the pontine reticular formation and propagate through the lateral geniculate nucleus of the thalamus, and then to the occipital cortex.
• These pontine-geniculo-occipital (PGO) waves therefore provide a useful marker for the beginning of REM sleep, although the specific purpose of the PGO waves is not clear.
• Interestingly, the overall duration of REM sleep varies as a function of age (Fig. 26.4).
• High proportion of fetal and infant sleep is REM sleep - which has provoked a good deal of speculation, that learning is linked to REM sleep.
• Most of the dreams occur during REM sleep. What is the function of dreams any way?
• An imaginative hypothesis by Francis Crick is: The function of dreams is to act as an "unlearning" mechanism, whereby certain modes of neural activity are erased by random activation.
• Roughly speaking, unwanted thoughts or erroneous information, which, if not expunged, might become the basis for obsession, paranoia, or other thought pathology
• Although REM sleep occurs in nearly all mammals. Birds, reptiles and amphibians lack it.
• Even few mammals like the spiny anteater, which has a remarkably large cerebral cortex, does not show any signs of REM sleep. (the big brain may be to compensate the inability to expunge parasitic mode of thoughts).
• Deprivation of REM sleep in humans for as much as 2 weeks has little or no effect on behavior.
• Similarly, patients taking certain antidepressants have little or nor REM sleep, yet show no obvious ill effects, even after months or years of treatment.

Biological Clocks (P. 504)

• Human sleep occurs with circadian periodicity.
• What happens, for example, if one is prevented from sensing cues they normally have about night and day?
• The normal circadian rhythm is maintained, but slightly loses its usual relationship to the actual time. For example, a day becomes 25 hours, instead of the normal 24 (Fig. 26.7 A).
• Thus, humans (and many mammals) have an internal clock that continues to operate in the absence of any external information about the time of day.
• In animals, this clock is the suprachiasmatic nucleus (SCN) of the hypothalamus (Fig. 26.7 B).
• Removal of SCN in animals abolishes their circadian rhythm.
• Recent findings suggest that cells in the SCN contain oscillatory molecular mechanisms that control expression of specific gene products, effectively creating a timing mechanism for circadian rhythm.
• Not surprisingly, SCN receives info from the retina, and is connected to the brain structures important in controlling sleep and wakefulness (see below).

Brainstem Mechanisms of Sleep and Wakefulness (P. 506)

• Sleep and wakefulness are now understood to be generated by the activation of specific neural centers.
• Electrical stimulation of the midbrain reticular formation, called reticular activating system (RAS) causes a state of wakefulness and arousal.
• RAS is a group of nuclei near the pons-midbrain junction (Fig. 26.8).
• Many of the neurons in these nuclei have high discharge rates during waking and in REM sleep; conversely they are quiescent during non-REM sleep.
• These nuclei have widespread ascending and descending connections to other regions of the brain, which explains their numerous effects.
• This observation implied that wakefulness requires a special mechanism, not just presence of adequate sensory experience.
• A quite different idea about the mechanism of sleep is that one or more molecules builds up in the brain during wakefulness and induce sleep. Although, not widely accepted, this idea has led to the identity of bacterial muramyl dipeptide as the sleep factor in humans (Box C).