The Drive to Sleep and Our Internal Clock

At a Glance

  • Finding sleep impossible even when you have the chance can be extremely frustrating, but this scenario is common among people who have jet lag or do shift work.
  • Under normal conditions, two systems inside the body interact to allow us to sleep and remain alert when we want to.
  • Understanding what happens when these two systems fall out of synch is an important step to achieving quality sleep even if you travel or work the night shift.

Sleep Drive

Nodding off at an inopportune moment can be embarrassing or even dangerous. And anyone who has experienced even a short bout of insomnia can attest to the frustration caused by the inability to sleep at a desired hour. These instances might happen more often if it weren't for two systems whose interaction governs wakefulness and sleep.

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 and functional 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.

Scientists refer to sleep drive as a homeostatic system. Like body temperature or blood sugar, sleep is regulated internally. For instance, 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 8:00 p.m. than we are at 3:00 p.m.

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The Forces that Control Sleep and Wakefulness

How do we stay awake throughout the day and sleep through the night?  See the forces that make this happen.

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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.

Dr. Czeisler

The Biological Clock and Sleep Homeostat (1:25)

Dr. Charles Czeisler describes the interaction between the internal biological clock and the sleep homeostat.

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Sleep Drive and Body Clock Through the Night

In the first half of the nightly sleeping period your sleep drive is still significant, and your alerting signal is declining rapidly. In normal circumstances, this means it is easy to maintain sleep. However, after approximately four hours of uninterrupted sleep the situation changes. Now that your sleep drive has decreased, the simple absence of an alerting signal is no longer sufficient to maintain sleep. At this point, the internal clock, which was promoting alertness during the day, begins to play an active role in sleep promotion by sending signals to parts of the brain that serve this function. In this way, the homeostatic sleep drive and the circadian system, when synchronized, interact to provide consolidated periods of both alertness and sleep.

Alertness, of course, varies for most people over the course of a day. For example, the grogginess that people often experience in the mid-afternoon, and commonly attribute to a heavy lunch or a dull meeting, is usually the result of a brief lull in the strength of the alerting signal. While sleep drive continues to climb, there is an hour or two each afternoon during which the alerting signal fails to keep pace, and alertness suffers as a result. Many cultures have incorporated this lull into their lives by making mid-afternoon naps, or siestas, part of the daily routine.

Another variation in alertness can be found near the end of the waking period, when the alerting signal is at its highest. Sleep experts refer to the period from 8:00 p.m. to 9:00 p.m. (for people who follow a fairly typical sleep/wake schedule) as the "forbidden hour for sleep" because most people find it next to impossible to fall asleep between these times.

A Delicate Balance

It is clear that synchronization of the sleep wake schedule and the internal clock is essential to an individual's ability to maintain sleep and wakefulness when desired. This has been shown conclusively in sleep research and is widely supported by anecdotal evidence from people who fly across time zones or work night shifts. Both of these activities desynchronize sleep and wake patterns from the internal clock's circadian rhythms and result in an alerting signal that is too low when an individual wishes to be awake and too high to allow for a consolidated period of sleep.

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This content was last reviewed on December 18, 2007