Sleep
Although environmental factors affect sleep, there is evidence that sleep is genetically controlled. This is shown by sleep dis- orders that run in families and the heritability of sleep patterns. Histamine and several other neurotransmitters promote wake- fulness, while adenosine and GABA promote sleep. The neural control of sleep and arousal is discussed in conjunction with the reticular activating system in section 8.4.
Two categories of sleep are recognized. Dreams—at least those that are vivid enough to recall upon waking—occur dur- ing rapid eye movement (REM) sleep. The name describes the characteristic eye movements that occur during this stage of sleep. The remainder of the time sleeping is spent in non-REM, or resting, sleep. These two stages of sleep can also be distin- guished by their EEG patterns. The EEG pattern during REM sleep consists of theta waves (5 to 8 cycles per second), although the EEG is often desynchronized as in wakefulness. Non-REM sleep is divided into four stages based on the EEG patterns; stages 3 and 4 are also known as slow-wave sleep, because of their characteristic delta waves (1 to 5 cycles per second).
When people first fall asleep, they enter non-REM sleep of four different stages, and then ascend back through these stages to REM sleep. After REM sleep, they again descend through the stages of non-REM sleep and back up to REM sleep. Each of these cycles lasts approximately 90 minutes, and a person may typically go through about five REM-to-non-REM cycles a night. A great amount of time is spent in slow-wave sleep during the first half of a night’s sleep; this gives way to mostly REM sleep during the second half of the sleep. When people are allowed to wake up naturally, they generally awaken from REM sleep.
Most neurons decrease their firing rate in the transi- tion from waking to non-REM sleep. This correlates with a decreased energy metabolism and blood flow, as revealed by PET studies. By contrast, REM sleep is accompanied by a higher total brain metabolism and by a higher blood flow to selected brain regions than in the waking state. Interestingly, the limbic system (described shortly) is activated during REM sleep. The limbic system is involved in emotions, and part of it, the amygdala, helps to mediate fear and anxiety. Because these are common emotions during dreaming, it makes sense that the limbic system would be active during REM sleep.
During non-REM sleep, the breathing and heart rate tend to be very regular. In REM sleep, by contrast, the breathing and heart rate are as irregular as they are during waking. This may relate to dreaming and the activation of the brain regions involved in emotions during REM sleep.
Non-REM sleep aids the neural plasticity required for learning. For example, subjects allowed to have non-REM sleep after a learning trial displayed improved performance
Although environmental factors affect sleep, there is evidence that sleep is genetically controlled. This is shown by sleep dis- orders that run in families and the heritability of sleep patterns. Histamine and several other neurotransmitters promote wake- fulness, while adenosine and GABA promote sleep. The neural control of sleep and arousal is discussed in conjunction with the reticular activating system in section 8.4.
Two categories of sleep are recognized. Dreams—at least those that are vivid enough to recall upon waking—occur dur- ing rapid eye movement (REM) sleep. The name describes the characteristic eye movements that occur during this stage of sleep. The remainder of the time sleeping is spent in non-REM, or resting, sleep. These two stages of sleep can also be distin- guished by their EEG patterns. The EEG pattern during REM sleep consists of theta waves (5 to 8 cycles per second), although the EEG is often desynchronized as in wakefulness. Non-REM sleep is divided into four stages based on the EEG patterns; stages 3 and 4 are also known as slow-wave sleep, because of their characteristic delta waves (1 to 5 cycles per second).
When people first fall asleep, they enter non-REM sleep of four different stages, and then ascend back through these stages to REM sleep. After REM sleep, they again descend through the stages of non-REM sleep and back up to REM sleep. Each of these cycles lasts approximately 90 minutes, and a person may typically go through about five REM-to-non-REM cycles a night. A great amount of time is spent in slow-wave sleep during the first half of a night’s sleep; this gives way to mostly REM sleep during the second half of the sleep. When people are allowed to wake up naturally, they generally awaken from REM sleep.
Most neurons decrease their firing rate in the transi- tion from waking to non-REM sleep. This correlates with a decreased energy metabolism and blood flow, as revealed by PET studies. By contrast, REM sleep is accompanied by a higher total brain metabolism and by a higher blood flow to selected brain regions than in the waking state. Interestingly, the limbic system (described shortly) is activated during REM sleep. The limbic system is involved in emotions, and part of it, the amygdala, helps to mediate fear and anxiety. Because these are common emotions during dreaming, it makes sense that the limbic system would be active during REM sleep.
During non-REM sleep, the breathing and heart rate tend to be very regular. In REM sleep, by contrast, the breathing and heart rate are as irregular as they are during waking. This may relate to dreaming and the activation of the brain regions involved in emotions during REM sleep.
Non-REM sleep aids the neural plasticity required for learning. For example, subjects allowed to have non-REM sleep after a learning trial displayed improved performance
Alpha
Beta
Theta
Delta
have the highest amplitude and lowest frequency.
Beta
Theta
Delta
have the highest amplitude and lowest frequency.
1 sec
Different types of waves in an electro- encephalogram (EEG). Notice that the delta waves (bottom)
Different types of waves in an electro- encephalogram (EEG). Notice that the delta waves (bottom)
compared to those who were not allowed to have non-REM
sleep. In another study, slow-wave activity in an EEG (indi-
cating non-REM sleep) increased in trained subjects, and the
magnitude of that increase correlated with how well the sub-
jects performed on the learned task the next morning.
These and other studies demonstrate that, although short- term memory is formed while a person is awake, the consolida- tion of short-term into long-term memory is promoted by sleep. Slow-wave sleep particularly benefits the consolidation of spatial and declarative memories (those that can be verbalized). REM sleep has been shown to benefit the consolidation of nondeclara- tive memories, but evidence suggests that both stages of sleep may participate in the consolidation of declarative and nonde- clarative memories. Indeed, memory consolidation is best when slow-wave and REM sleep phases follow each other naturally.
Memory consolidation improves after a nap, but longer durations of sleep are required for maximum benefit. Evidence suggests that the time delay between the learning session and sleep is also an important consideration. Experiments indicate that a time delay of about three hours between a learning ses- sion and sleep provides better declarative memory consolidation than a delay of eight hours. These studies strongly suggest that students would improve their performance on an exam if they studied earlier and got a good night’s sleep before the exam.
These and other studies demonstrate that, although short- term memory is formed while a person is awake, the consolida- tion of short-term into long-term memory is promoted by sleep. Slow-wave sleep particularly benefits the consolidation of spatial and declarative memories (those that can be verbalized). REM sleep has been shown to benefit the consolidation of nondeclara- tive memories, but evidence suggests that both stages of sleep may participate in the consolidation of declarative and nonde- clarative memories. Indeed, memory consolidation is best when slow-wave and REM sleep phases follow each other naturally.
Memory consolidation improves after a nap, but longer durations of sleep are required for maximum benefit. Evidence suggests that the time delay between the learning session and sleep is also an important consideration. Experiments indicate that a time delay of about three hours between a learning ses- sion and sleep provides better declarative memory consolidation than a delay of eight hours. These studies strongly suggest that students would improve their performance on an exam if they studied earlier and got a good night’s sleep before the exam.
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