Dual Forces of Sleep: Adenosine & Circadian Rhythm

The Dual Forces That Determine Sleep and Wakefulness

Walker explains that two main forces determine when we want to sleep and wake: the circadian rhythm and sleep pressure.

The circadian rhythm is a master clock in the brain called the suprachiasmatic nucleus that regulates activity and inactivity over a 24-hour period. Other cells in the body also have circadian rhythm clocks, but the central brain clock rules them all.

Sleep pressure, on the other hand, is determined by a chemical called adenosine that builds up in the brain from the moment we wake up. The more adenosine accumulates, the sleepier we feel. After about 16 hours of being awake, the sleepiness becomes powerful enough to make us want to go to bed.

Normally, the circadian rhythm and sleep pressure align in a harmonious dance, even though they are independent of each other. When the circadian rhythm is on its downward swing in the evening and sleep pressure is at its peak, we feel sleepy and go to sleep. During sleep, the brain clears away the adenosine over a period of seven to nine hours. By the time we naturally wake up, the adenosine is gone and the circadian rhythm is on its upward swing, making us feel alert and ready to start the day.

The Two Forces of Sleep: Adenosine and Circadian Rhythm

Walker explains the complex interplay between the circadian rhythm and adenosine levels in the brain during sleep deprivation. When an individual is deprived of sleep for 24 hours, adenosine levels continue to rise, making them feel increasingly tired. However, around 11:00 a.m., the circadian rhythm begins its upswing, counteracting the effects of adenosine and providing a temporary boost in alertness.

As the day progresses and the individual remains awake, adenosine levels continue to build, while the circadian rhythm enters its downswing by early evening. This combination of factors makes it extremely difficult to stay awake, as the body is overwhelmed by the need for sleep.

Walker emphasizes that the circadian rhythm and adenosine levels operate independently of each other. The circadian rhythm continues its 24-hour cycle regardless of adenosine levels, while adenosine accumulates in the brain as long as the individual remains awake, only dissipating during sleep.

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