Circadian rhythm

Regulated by the suprachiasmatic nucleus in the hypothalamus.

Is observed in body temperature, heart rate, secretion of several hormones, red blood cell production and other physiological characteristics.

The intrinsic clock determines sleep-wake cycles, cognitive functions such as learning and memory, daily variations in blood pressure, heart rate, hormone levels, respiratory and exercise capacity, and coagulation.

The intrinsic circadian clock is slightly longer than 24 hours, which is different than the 24 hour solar day and the circadian clock resets to compensate for the difference.

Light is the most important time cue for synchronizing circadian rhythms.

Exposure to light in the evening shifts the clock to a later time, while exposure to light in the morning shifts the clock to an earlier time (Khalsa SB).

During the night there is a crossover point separating evening responses to light exposure from morning responses.

Sleep limits the exposure to light and plays an important role in the regulation of the circadian clock.

Sleep disturbances occur with disruption of circadian rhythm.

Many pathological events occur at specific times of the day suggesting that circadian processes contribute to disease.
The circadian rhythm system drives periods of energy acquisition and is used in anticipation of the cycling of day and night.
Circadian rhythms persist even under constant conditions, with a period of almost 24 hours in duration.
Circadian rhythms have a value, and are critical for fitness and survival of species.
Pacemaker neurons exist that warehouse circadian clocks driving sleep-wake rhythms, and orchestrating such clocks in peripheral tissues.
The hypothalamic suprachiasmatic nucleus (SCN) comprises about 20,000 neurons and glia, containing the pacemaker neurons necessary to drive circadian rhythms.
These circadian pacemaker neurons have high-amplitude day-night variation in their spontaneous firing rate and resting membrane potential.
Sodium and potassium currents within pacemaker neurons, and spike in associated conductance, confer excitatory and inhibitory drives, respectively, for activity rhythms.

Alzheimer’s disease leads to pathological changes in the suprachiasmatic nucleus and disruption in circadian rhythms.

Asthma has an increased incidence of status asthmaticus during the early hours of the morning.

Morning stiffness characteristic of rheumatoid arthritis.

Osteoarthritis typically have worse symptoms in the afternoon.

Symptoms of allergic rhinitis typically more severe in the early morning hours.

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