The Anatomy and Sleep Functions of the Brain's Suprachiasmatic Nucleus

Brain Area Controls Timing of Sleep, Wakefulness, Temperature, and Hormones

The human circadian rhythm is controlled by light exposure affecting the suprachiasmatic nucleus (SCN) of the brain's hypothalamus
The human circadian rhythm is controlled by light exposure affecting the suprachiasmatic nucleus (SCN) of the brain's hypothalamus. Universal Images Group / Getty Images

What is the suprachiasmatic nucleus (SCN)? Learn about the anatomy of this brain region in the hypothalamus and how the SCN controls the body's sleep and wake patterns as well as temperature, metabolism, and hormone release. Discover what happens if the SCN becomes damaged and what other signals may become important.

A Word on Circadian Rhythms

There are multiple internal processes that are externally influenced by our environment.

Earth has a 23 hour and 56 minute daily rotation that provides predictable patterns of light and temperature, food, and predator activity. Through adaptive evolution, these patterns influence the metabolism and behavior of nearly every organism on the planet, including humans. The term circadian describes the approximately 24-hour cycles that are generated independent of these external influences. In humans, some of the precisely timed internal processes that can persist in isolation include:

  • Sleep and wakefulness
  • Metabolism
  • Core body temperature
  • Cortisol levels
  • Melatonin levels
  • Other hormones

There is an interplay between the internal body clock and external time cues. Many tissues in the body will follow a circadian pattern, with the primary coordinator being an area of the brain called the suprachiasmatic nucleus.

What Is the Suprachiasmatic Nucleus of the Brain's Hypothalamus?

The suprachiasmatic nucleus is located in a foreward region of the brain called the hypothalamus.

It contains a group of nerve cells (or neurons) that control your body's circadian rhythm. The suprachiasmatic nucleus lies in a shallow impression of the optic chiasm, where the nerves extending from each eye to the brain cross, and thus it is highly influenced by the input of light.

The Role of the SCN in Normal Sleep and Circadian Rhythms

Light is the predominant controller of the body's circadian rhythms.

It is detected in the eye by receptors called intrinsically photosensitive retinal ganglion cells (ipRGC) which contain the photopigment melanopsin. There is a connection called the retinohypothalamic tract that extends from the retina of the eye to the anterior hypothalamus. Within the hypothalamus sits the SCN, the body's central pacemaker (or master clock).

Circadian rhythm sleep disorders can occur when the synchrony between the body and the external environment is lost. Patterns of sleep and wakefulness may no longer align with societal norms. These conditions include:

These conditions occur for multiple reasons. Some people with delayed sleep phase are genetically programmed to have a circadian misalignment, with the condition running in families and starting in adolescence. These problems may be worsened by lifestyle choices, including irregular sleep patterns or inadequate or poorly timed light exposure.

It is also possible for a degenerative condition, injury, or other causes to contribute to the development of a circadian disorder.

What Happens if the Suprachiasmatic Nucleus is Damaged?

In considering the pathway from light perception to the SCN, it is possible for damage to occur at multiple locations. People who are totally blind, completely lacking the perception of light, can develop Non-24. In the setting of degenerative conditions like Alzheimer's disease, especially those who live in an institution like a nursing home, an irregular sleep-wake rhythm may develop. Trauma, stroke, or tumors may also impact the SCN and cause its dysfunction.

When the central pacemaker of the body is damaged and its function becomes compromised, the peripheral clocks have lost their director. The timing of hormone release, metabolism, and other processes may become disturbed. There is some early research that suggests this may contribute to various disease states.

Other signals for timing may become important. In the blind, a low dose of melatonin in the evening may be necessary. Alternatively, the use of Hetlioz may be indicated. The timing of social activities, exercise, meals, environmental temperature, and the bedroom environment may take a more prominent role in influencing the timing of internal processes.

If you are concerned that you may have a circadian disorder, start by speaking with a sleep specialist. You can review your symptoms and discover effective treatments that will help you to sleep better at night and feel better during the day.

Sources:

Honma, K et al. “Differential effects of bright light and social cues on reentrainment of human circadian rhythms.” Am J Physiol. 1995;268:R528-R535.

Kryger, MH et al. "Principles and Practice of Sleep Medicine." ExpertConsult, 5th edition, 2011.

Moore-Ede, MC et al. “A physiological system measuring time, “ in The Clocks That Time Us. Cambridge, Massachusetts, Harvard University Press, 1984, p. 3.

Peters, BR. “Irregular Bedtimes and Awakenings.” Evaluation of Sleep Complaints. Sleep Med Clinic. 9(2014)481-489.

Ramsey, KM and Bass, J. “Animal models for disorders of chronobiology: cell and tissue,” in Principles and Practices of Sleep Medicine. Edited by Kryger MH, Roth T, Dement WC. St. Louis, Missouri, Elsevier Saunders, 2011, pp. 463-467.

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