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Neurophysiology
The thalamocortical pathways synchronize cortical activity, particularly during the deeper stages of NREM sleep. The cortex is in effect removed from the influence of ascending sensory input by the thalamic 'gate' and also by reduction in the activity of the ascending reticular activating system. This disconnection contrasts with wakefulness and REM sleep (Table 2.1).

The synchronization of the cortical activity is not homogeneous. Early in NREM sleep it is most marked in the prefrontal area, suggesting that this region, which is particularly active during wakefulness, develops a greater homeostatic drive to enter NREM sleep. This regional difference is particularly marked after sleep deprivation.

Thalamic activity in NREM sleep stabilizes this state and inhibits arousals. Arousal stimuli may however lead to a reduction in delta activity and an increase in alpha and beta rhythms. The threshold for painful stimuli causing arousal increases from stage 1 through to stage 4 NREM sleep. Impulses ascending in the spinoreticular tract can be inhibited in the nucleus reticularis gigantocellularis so that afferent stimuli are prevented from causing arousals.

Functional neuro-imaging
Functional neuro-imaging has revealed global shifts in cerebral function between wakefulness and NREM sleep, particularly stages 3 and 4. In this state there is reduced activity in the brainstem, particularly the pons and cerebellum, in the basal forebrain and limbic cortex, such as the anterior cingulate gyrus, and especially in the dorsolateral prefrontal and inferior parietal cortex. There is little change in activity relative to wakefulness in the basal ganglia and primary sensory and motor cortex.

On waking from NREM sleep there is an increase in brainstem activity, followed by increasing activity in the cerebral cortex. The prefrontal cortex is the last to be activated after waking, and this sequence may explain the features of sleep inertia. Activity patterns midway between normal wakefulness and NREM sleep are seen during wakefulness following sleep deprivation. Wakefulness in the evening is associated with increasing brainstem and hypothalamic metabolic activity, possibly due to increased input from the circadian rhythm generators to maintain wakefulness.

Electroencephalogram (EEG) activity
The depth of NREM sleep can be quantified by electroencephalogram (EEG) fluctuations. Four stages of NREM sleep are recognized on conventional EEG criteria. These are categorized by the frequency and amplitude of the EEG, the presence of sleep spindles and K-complexes, and electro-oculogram (EOG) and electromyogram (EMG) findings.

Mental activity
Subjects woken during NREM sleep frequently report awareness of fragmented, thought-like processes, particularly in the lighter stages and later in the night.

These contain less action than the dreams that occur during REM sleep. The simpler dream mentation in NREM sleep is probably due to the functional disconnection of the cortex from sensory input by the thalamus. This may represent a process of reorganization of neural networks in an analogous, but different, manner to that which occurs in REM sleep.