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One of the important questions in sleep research is clearly defining the sleep state. This problem arises because sleep was traditionally defined as a state of consciousness and not as a physiological state, [14] [15] thus there was no clear definition of what minimum set of events constitute sleep and distinguish it from other states of partial or no consciousness.
Faster rhythms such as gamma activity have been linked to cognitive processing. Indeed, EEG signals change dramatically during sleep. In fact, different sleep stages are commonly characterized by their spectral content. [16] Consequently, neural oscillations have been linked to cognitive states, such as awareness and consciousness. [17] [18 ...
Theta rhythm is prominent during part of awaking and REM sleep. Due to the density of its neural layers, the hippocampus generates some of the largest EEG signals of any brain structure. In some situations the EEG is dominated by regular waves at 4–10 Hz, often continuing for many seconds.
Gamma oscillations have also been shown to correlate with the firing of single neurons, mostly inhibitory neurons, during all states of the wake-sleep cycle. [15] Gamma wave activity is most prominent during alert, attentive wakefulness. [13]
They can be predominantly recorded from the occipital lobes during wakeful relaxation with closed eyes and were the earliest brain rhythm recorded in humans. [3] Alpha waves are reduced with open eyes and sleep, while they are enhanced during drowsiness. Occipital alpha waves during periods of eyes closed are the strongest EEG brain signals. [4]
The core body and brain temperatures increase during REM sleep and skin temperature decreases to lowest values. [1] The REM phase is also known as paradoxical sleep (PS) and sometimes desynchronized sleep or dreamy sleep, [2] because of physiological similarities to waking states including rapid, low-voltage desynchronized brain waves.
Sharp waves and ripples (SWRs) are oscillatory patterns produced by extremely synchronised activity of neurons in the mammalian hippocampus and neighbouring regions which occur spontaneously in idle waking states or during NREM sleep. [1] They can be observed with a variety of imaging methods, such as EEG.
It is most readily observed in stages of sleep, for example, rapid eye movement sleep (REM) and the delta activity cycle. When awake, brainwaves are faster during the first half of the cycle which corresponds to feeling alert and focused. During the last 20 minutes brainwaves slow and as the body feels dreamy or tired.