Understanding Sleep – The Hypothalamus

By Sheila Kun RN, BSN, MS, CPN, FCCP

I have a confession to make; last week, we detailed the anatomy of the brain center that governs sleep. I have a feeling that each structure is pretty novel to you. I, therefore, take the liberty of dissecting each important part of the brain that regulates sleep and try to present the information slowly. To begin with, we will start with the hypothalamus.

Let us review the write up from last week:

The hypothalamus, a peanut-sized structure deep inside the brain, contains groups of nerve cells that act as control centers affecting sleep and arousal.  Within the hypothalamus is the suprachiasmatic nucleus (SCN) – clusters of thousands of cells that receive information about light exposure directly from the eyes and control your behavioral rhythm.  Some people with damage to the SCN sleep erratically throughout the day because they are not able to match their circadian rhythms with the light-dark cycle.  Most blind people maintain some ability to sense light and are able to modify their sleep/wake cycle.

From the Sleep Health from Harvard Medicine, the role of the suprachiasmatic nucleus (SCN) is further explained. The timing of transitions between sleep and wakefulness are also tied closely to the body’s internal biological clock located in the suprachiasmatic nucleus (SCN). This tiny structure—made up of approximately 50,000 brain cells—receives light signals directly from the eye, through the optic nerve. Light resets the clock to correspond to the day-night cycle. In turn, the clock regulates the timing of dozens of different internal functions, including temperature, hormone release, and sleep and wakefulness. The SCN promotes wakefulness by producing a powerful alerting signal that offsets sleep drive. The SCN promotes sleep by turning off the alerting signal. In addition, the SCN actively maintains sleep throughout the night even after sleep drive has dissipated in the second half of the night.

Neurons in a part of the hypothalamus called the ventrolateral preoptic nucleus (VLPO) connect directly to the many arousal-promoting centers. Rather than stimulating activity in these areas, signals from VLPO neurons inhibit their activity. By shutting down the arousal centers, the VLPO promotes sleep.

To summarize the above discussion, it seems like the suprachiasmatic nucleus (SCN) in the hypothalamus turns off alerting signals, allowing sleep to occur. Similarly, the ventrolateral preoptic nucleus (VLPO) also can inhibit their activity and turn off the arousal centers, promoting sleep to occur.

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Your homework from the Care Ministry this week: appreciate the role of the hypothalamus in allowing sleep to occur.

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