Homeostatic Sleep Drive

The Desire to Sleep Gradually Increases with Prolonged Wakefulness

Businessman napping on train
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Homeostatic sleep drive refers to the desire to sleep that gradually increases with prolonged wakefulness. It is lessened during sleep. This equilibrium is one of the primary determinants of sleep. For example, if one stays up for 3 days, the homeostatic sleep drive will cause profound sleepiness.

Homeostatic sleep drive is also known as sleep debt or sleep load.

We have all experienced that undeniable drive to sleep. Staying up much later than usual, or rising after only a few hours of sleep and then attempting to stay alert throughout the day, serve as an unpleasant reminder of the power of the sleep drive. And even when we feel alert and are unaware of our sleep drive, it is always present and growing while we're awake. In fact, the only true way to reduce rather than mask sleep drive is to sleep. 

A Homeostatic System

Scientists refer to sleep drive as a homeostatic system.

Similar to body temperature or blood sugar, sleep is regulated internally. For example, when body temperature falls, blood vessels constrict and we shiver; when blood sugar levels rise, the pancreas secretes insulin; and when we remain awake for an extended period of time, structures in the brain promote sleep. Furthermore, the duration and depth of our sleep vary according to the quantity and quality of sleep obtained previously.

With every waking hour there is a strengthening of the homeostatic sleep drive. This strengthening isn’t directly measurable as a quantity, but experts think that it is the result of the level of brain activity during wakefulness. One hypothesis suggests that the build-up in the brain of  adenosine, a by-product of energy consumption by cells, promotes sleep drive. The fact that both adenosine and sleep drive increase during wakefulness and dissipate during sleep suggests a possible link between the two.

Awake and Asleep

Homeostatic sleep drive is not the only force involved in regulating the transition from wakefulness to sleep. If it were, catnapping throughout the day and night would likely be the norm rather than the exception. After just a few hours awake, we might nod off for an hour and then rise again, only to succumb to sleep just a few hours later.

Instead, most of us remain awake—and alert—for 16 hours or more each day without respite. And despite the fact that our sleep drive increases with every hour of wakefulness, we are typically no sleepier at 8 p.m. than we are at 3 p.m.

Circadian Alerting System

Our relatively steady state of alertness over the course of a 16-hour day is due to what scientists call the circadian alerting system, a function of our internal biological clock. The clock, which is responsible for regulating a vast number of daily cycles, is found in a relatively small collection of neurons deep within the brain. Under normal conditions, the clock is highly synchronized to our sleep/wake cycle. When it is, the clock's alerting signal increases with every hour of wakefulness, opposing the sleep drive that is building at the same time. Only when the internal clock's alerting signal drops off does sleep load overcome this opposing force and allow for the onset of sleep.

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