Circadian Rhythm

The sleep-wake cycle is coupled with the external rotation of light and dark and follows a 24-hours pattern or Circadian Rhythm. It may become uncoupled in free-running conditions when there are no external cues, showing a tendency to shift to a longer duration.

Sleep tends to emerge in the late evening when the lights are dim and the core body temperature begins to decline. During sleep, most hormones show individual patterns of secretion that intervene importantly in the homeostasis of body systems. Two distinct states of being dominate brain activity during sleep: NREM sleep is associated with quiescence, repose, low body temperature, and hypometabolism; and REM sleep is a state of neural instability and high cerebral activity that recurs at 90 minute intervals and dominates the second half of the night. Uncoupling of these two states of being from each other and from other Circadian Rhythms normally linked to them leading to pathology that reduces the quality of both the awake and sleep states.

The ultimate function or function or functions of sleep remain unknown. The intuitive exploration indicates that the repose and quiescence of sleep are necessary to restore vital energies and the powers of the body. This rationale fails to take into account the periodic output and rich variety of high-amplitude brain electrical activity that manifests a transfer to a set of systems different from wakefulness. And yet restoration must be a significant component of the programmed functions, since successful sleep is followed by sensations of satisfaction and replenishment. Most neurologic deficits tend to get worse by day’s end when the individual is tired and appears improved in the morning.

Unraveling the enigma of sleep function is important for all neuroscience. Several new testable ideas concerning sleep functions exist. Many of these theories demand a new look at sleep mechanisms, brain organization and reinterpretation of the past sleep literature.

Mechanism of Sleep and Wakefulness

Humans have three sleep-wake states:

1. Waking

2. Asleep (Resting or slow-wave sleep)

3. Asleep and dreaming (paradoxical, active or rapid eye movement (REM sleep)

These three states occur in a relatively predictable manner. These states are controlled by networks in the brainstem, hypothalamus, basal forebrain and thalamocortical systems, where they are regulated by different but interrelated neurochemical systems. Sufficient information exists to explain these states according to the firing properties of neurons in these brain regions based on their intrinsic membrane properties, synaptic and neurochemical correctivity and responsiveness to sensory inputs.

The three sleep-wake states in the human (waking, resting sleep and REM sleep) are generated by both evolutionarily – conserved and new nervous system networks.

The rhythms generated during each state can be ascribed to known cell groups that possess neurons with the appropriate membrane properties, which have organized synaptic relationships and well-matched neurochemical interactions. Networks in the brainstem, hypothalamus, basal forebrain, and thalamocortical systems modulate the synchronization of fast rhythms during waking and REM sleep and the synchronization of slow rhythms during resting sleep.

Respiratory Control During Sleep

Control of respiration differs significantly between sleep and wakefulness. However, the respiratory system is designed to maintain homeostasis of PO2, PCO2, and PH regardless of the state of consciousness, and homeostasis of blood gases is generally well maintained during sleep, although not at waking values. Sleep modifies the mechanical chemical, and neural processes controlling ventilation and two significant factors that influence respiration during wakefulness are removed during sleep: One is the effect of wakefulness or respiratory control, and the other is volitional control of ventilation. Moreover, sleep is not a homogeneous state, and the effects of sleep or respiratory control are not homogeneously distributed across all sleep states.