Sleep Timing

Sleep timing is controlled by three main factors and they are the circardian clock, homeostasis and in humans by willed behavior.

The circadian clock is an inner time-keeping, temperature-fluctuating, enzyme-controlling device, which works in tandem with adenosine, a neurotransmitter which inhibits many of the bodily processes that are associated with wakefulness.

Adenosine is generated during the day and increased levels of adenosine leads to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one's chronotype. It is the circadian rhythm which determines the ideal timing of a correctly structured and restorative sleep episode.

Homeostatic sleep propensity is the need for sleep as a function.  The amount of time elapsed since the last adequate sleep episode is an important factor and must be balanced against the circadian element for satisfactory sleep. This along with corresponding messages from the circadian clock, informs the body that it needs to sleep. Sleep offset, awakening, is primarily determined by circadian rhythm. A normal person who normally wakes up at an early hour will not be able to sleep much later than a person who wakes up at his normal waking time, even though he or she is moderately sleep deprived.

The association of circadian rhythms and sleep is clear and further research and development in genetics and molecular biology will add more clarity and scientific evidence to functions of the circadian clock and how behavioral patterns will evolve and adjust to the light and dark cycles. 

In addition to the circadian component, there is a fundamental regulatory process involved in programming sleep. Sleep deprivation and the need for sleep are almost directly proportional to each other, for instance, it is quite evident and conclusive that the more a person is deprived of sleep the more he or she needs to sleep. This need to sleep is the definition of the hemostatic component of sleep.  The precise mechanism that generates this pressure and makes us feel the need to sleep remains a mystery. However we have established that the action of nerve-signaling molecules called neurotransmitters and nerve cells (neurons) are located in the brainstem and at the base of the brain.  These neurotransmitters and nerve cells determine whether we are asleep or awake.

Additionally, there is recent evidence that the molecule adenosine (composed of the base adenine linked to the five-carbon sugar ribose) is an important factor that is responsible for sleepiness and it appears to “keep track” of lost sleep and may induce sleep. Interestingly, caffeine binds to and blocks the same cell receptors that recognize adenosine. This suggests that caffeine disrupts sleep by binding to adenosine receptors and preventing adenosine from delivering its fatigue signal. The homeostatic regulation of sleep helps reinforce the circadian cycle. We usually sleep once daily because the homeostatic pressure to sleep is hard to resist after about 16 hours, and then while we sleep, our closed eyes block the light signals to the biological clock.

The biological clock that times and controls a person’s sleep/wake cycle will attempt to function according to a normal day/night schedule even when that person tries to change it. Those who work night shifts naturally feel sleepy when nighttime comes. A similar feeling that occurs during travel is known as jet lag.  During jet lag there is a conflict created by trying to be active during the brain’s biological nighttime, which leads to a decrease in cognitive and motor skills. The biological clock can be reset, but only by appropriately timed cues and even then, by one to two hours per day at best.  Problems resulting from a mismatch of this type may be reduced by following some basic rules like sleeping in a dark, quiet room, and getting exposure to bright light at the right time, and altering eating and exercise patterns. Because humans function best when they sleep at night and act in the daytime, the task for a person who must be active at night is to retrain the biological clock (by light cues).

An internal biological clock regulates the timing for sleep in humans. The activity of this clock makes us sleepy at night and helps us stay awake during the day. Our clock cycles for a 24-hour period and is called a circadian clock (from the Latin roots circa = about and diem = day). In humans, this clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus in the brain. The SCN is actually a very small structure consisting of a pair of pinhead-size regions, each containing only about 10,000 neurons out of the brain’s estimated 100 billion neurons.

Biological clocks are genetically programmed-physiological systems that allow organisms to live in harmony with natural rhythms, such as day/night cycles and the changing of seasons. The most important function of a biological clock is to regulate overt biological rhythms like the sleep/wake cycle. The biological clock is also involved in controlling seasonal reproductive cycles in some animals through its ability to track information about the changing lengths of daylight and darkness during a year.

Biological rhythms are of two general types, exogenous rhythms and endogenous rhythms. Exogenous rhythms are directly produced by an external influence, such as an environmental cue. The organism itself does not generate exogenous rhythms internally, and if the environmental cues are removed, this rhythm ceases. Endogenous rhythms, however, are driven by an internal, self-sustaining biological clock rather than by anything external to the organism. Biological rhythms, such as oscillations in core body temperature, are endogenous. They are maintained even if environmental cues are removed.

In general, the human circadian clock appears to be better equipped to adjust to a longer day than a shorter day. For example, it is easier for most people to adjust to the end of daylight savings time in the fall when we have one 25-hour day than to the start of daylight savings time in the spring, when we have a 23-hour day. Similarly, traveling from the West Coast of the United States to the East Coast produces a loss of three hours—a 21-hour day. Thus, travelers may find it difficult to sleep because of the three-hour difference between external cues and their internal clock. Likewise, travelers may find it difficult to awaken in the morning. We may try to go to sleep and wake up at our usual local times of, say, 11 p.m. and 7 a.m., but to our brain’s biological clock, the times are 8 p.m. and 4 a.m. This particular lag, which people experience when they travel to and from different time zones, is referred to as Jet Lag.

Jet lag is a direct consequence caused due to the inability of our circadian clock to make a quick adjustment to the changes in light cues that come from a rapid change in time zone. After such travel, the body is in conflict. The biological clock carries the rhythm entrained by the original time zone, even though the clock is out of step with the cues in the new time zone. This conflict between external and internal clocks and signals is called desynchronization and it affects more than just the sleep/wake cycle. All the rhythms are out of sync, and they take a number of days to re-entrain to the new time zone.

When daily sleep time is less than an individual needs, a “sleep debt” develops. Even relatively modest daily reductions in sleep time (for example, one hour) can accumulate across days to cause a sleep debt. If the debt becomes too great, it can lead to problem sleepiness. Although the individual may not realize his or her sleepiness, sleep debt can have powerful effects on daytime performance, thinking, and mood.

Excerpts taken from this article are licensed under the GNU Free Documentation License. They use material from Wikipedia topics "Lucid Dream" and/or "Sleep".