The suprachiasmatic nucleus (SCN) of the hypothalamus regulates circadian rhythms. Light activates intrinsically photosensitive retinal ganglion cells, causing the release of glutamate onto SCN neurons. This eventually leads to the transcription of the circadian regulating genes Per1 and Per2. The SCN is referred to as the "circadian pacemaker" because it oscillates in a circadian fashion.

University of Michigan - Ann Arbor

An internal biological clock that allows animals to proceed with their circadian rhythms. Usually, the internal clock is dependent on light. However, animals will still continue their circadian rhythm even in the absence of external cues. For example, even if it is constantly light or dark, animals will proceed with their circadian rhythm. 
- free running

Endogenous clock and circadian rhythm

A circadian rhythm is a type of biological rhythm characterized by a cycle that lasts approximately 24 hours.

Circadian Rhythm

Luo, L. (2020). Principles of Neurobiology (2nd ed.). CRC Press. 

Principles of Neurobiology, 2nd edition (2020)

When animals continue to maintain their own cycle even when they aren't provided external cues about time of day, it is called free-running. The cycle during free-running is not exactly 24 hours. 
- changes in circadian rhythm
- hypothalamus 

Free running

The process by which circadian rhythms can be reset by light. 

* Phase delay: time of sleep and wake move to later in the day, due to light exposure in the first half of the night.
* Phase advance: time of sleep and wake move to earlier in the day, due to light exposure in the second half of the night..

Entrainment

Suprachiasmatic nucleus in circadian rhythms

An individual's level of alertness is correlated with their cyclical body temperature changes. Typically, periods of higher body temperature are associated with greater alertness, whereas lower body temperatures are linked to sleepiness.

Correlation Between Body Temperature and Alertness

Many physiological processes follow a 24-hour circadian rhythm. Notable examples include the sleep-wake cycle and daily variations in body temperature, heart rate, blood pressure, and blood sugar.

Examples of Circadian Rhythms

States of consciousness change across the day and across the lifespan, and biological rhythms are important contributors to those changes. Circadian rhythms are a specific biological rhythm involved in these shifts, and they are generated by the suprachiasmatic nucleus (SCN).

Role of Circadian Rhythms in Consciousness

The suprachiasmatic nucleus (SCN), a structure within the hypothalamus, functions as the brain's primary biological clock. It receives direct input about light levels from the retina, which allows it to synchronize the body's internal rhythms with the external day-night cycle. The SCN then orchestrates these rhythms by sending signals to other parts of the brain, including the pineal gland, to regulate physiological and behavioral outputs.

Suprachiasmatic Nucleus (SCN) as the Brain's Biological Clock

Sleep regulation refers to the brain's control over the timing and duration of sleep and wakefulness. This process is governed by the interaction of two primary systems: the circadian rhythm, which aligns sleep with the 24-hour environmental cycle, and homeostatic regulation, which builds sleep pressure based on the length of prior wakefulness.

Sleep Regulation

Based on your understanding of 24-hour biological cycles, explain the most likely reason for the office worker's symptoms of fatigue, sleep disruption, and altered appetite.

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