he evidence that diurnal rhythmicity modulates virtually every aspect of
mammalian physiology has forced reconsideration of historical precepts about homeostatic regulation of physiologic parameters.
Cycles of peaks of activity (and troughs of inactivity) occur at various
intervals (Figure 1). The cycle time between successive peaks (or troughs) is
called the period of the cycle and the extent of the increase (or decrease) is
called the amplitude. A period can be as short as a micro-second or as long as a
year (circannual rhythm). In humans, rhythms with a period of about 24 hours
(circadian) are of particular interest.
It is clear that peaks of activity of various physiological systems vary systematically with the time of day. Thus, the core body temperature of humans is not fixed at 98.6° Fahrenheit, but rather it is actively maintained near 100° at mid-afternoon and near 96° in the early morning hours before awakening. A similar pattern can be seen in plasma levels of hormones such as cortisol, growth hormone and prolactin as well as in urine production, heart rate, and blood pressure (Figure 2). Phylogenetic studies reveal that analogous daily variations, or diurnal rhythmicity, occur in almost all eukaryotic organisms from single-celled algae to humans.
The consistent presence of diurnal rhythmicity, both across species and within individual organisms, is strong evidence that variations in the activity of physiological systems confer an important adaptive advantage. Presumably this advantage derives from the optimization of energy expenditure and behavior in relation to the changing external environment. What is less clear from descriptions of diurnal rhythms is the origin of the drives or factors that promote the diurnal rhythmicity. Do rhythmic variations passively reflect the changing external environment-are they exogenously generated? Or does the organism anticipate the predictable features of daily environmental variations and provide its own rhythmicities, in anticipation-is there an endogenous generator?
The answer has been available since the late 18th century when the Swiss astronomer Jean-Jacques d'Ortous de Mairan first observed that diurnal leaf movements in plants persisted even when the plants were kept in a dark closet; when they were isolated from all environmental light or time signals. This finding and its implications remained largely unappreciated, however, until similar experiments were performed over 200 years later using nocturnal rodents housed in cages kept in continuous darkness.
Two important features common to all such diurnal rhythms are apparent in these sample data. First, it is clear that a rhythmic organization of activity persists even in the absence of environmental time cues, collectively termed zeitgebers. Second, in the absence of periodic time cues, the period length of the activity rhythm (the time from the onset of one bout of activity to the next) is significantly different from 24 hours. Instead, in rodents the period is significantly less than 24 hours, with onsets of activity lying along a line that deviates substantially from the vertical. This non-24 hour period length, which is a consistent feature of diurnal rhythms when observed in the absence of zeitgebers, is the origin of the term circadian (from circa meaning "approximately" and dies meaning "day").
The precise circadian period length varies from species to species and, to a lesser degree, between individual organisms within a given species. Entrainment to the 24 hour environmental cycle results from daily resetting in response to the zeitgeber signal. For example, an individual with an intrinsic period length of 25 hours needs a correction of one hour per day to be entrained to the 24 hour day. Entrainment involves synchronizing the period of the circadian rhythm to the period of the zeitgeber and the rhythm.
The persistence of a daily, i.e., circadian, rhythmicity in the absence of environmental time cues and the significant deviation of the circadian period from 24 hours provide convincing evidence that circadian rhythmicity arises from within the organism and that it is not a passive reflection of external environment influences. One important corollary of this conclusion is that somewhere in the organism there must be a circadian "clock" that is responsible for the generation of circadian rhythmicity.
The Search for the Circadian Clock
After several decades of research, it is now known that the principle mammalian circadian clock is located in the anterior hypothalamus. Lesioning specific nuclei within the hypothalamus has proven difficult because such lesions often directly disrupt the physiologic parameters, such as body temperature or activity, that are being used to measure circadian rhythmicity. However, in the late 70's experiments were conducted based upon the hypothesis that the prominent role of light in circadian entrainment implied a direct connection between the hypothalamic clock and the retina.
Ocular injections of tritiated amino acids were used to trace a previously unrecognized retinohypothalamic projection to the suprachiasmatic nuclei (SCN), which is located in the anterior hypothalamus on either side of the third ventricle immediately above the optic chiasm (Figure 3). It was then shown that destruction of the SCN rendered animals arrhythmic without any evidence of circadian organization (Figure 4). Next transplantation of fetal SCN into the brains of adult animals that had previously been rendered arrhythmic by SCN destruction resulted in the restoration of circadian rhythmicity. Thus, the circadian clock was discovered to reside in the suprachiasmatic nucleus of the hypothalamus.
Drawing of Nissl-stained section through the anterior hypothalamus (optic chiasm). The inset shows the plane of section.The amount of sleep and wakefulness does not significantly change when the SCN is destroyed, but the normal 24-hour rhythm of sleep and wakefulness is no longer present (Figure 4). Subsidiary clock-like or regulatory mechanisms that regulate specific processes are located in other areas of the central nervous system.
Left-hand diagram: normal 24 hour rhythm of a rat. The animal is active mainly at night (in darkness). Right-hand diagram: after elimination of the suprachiasmatic nucleus, the rhythm disappears completely. The thick horizontal lines indicate times when the rodent (a rat in this case) is running on a running wheel. Successive days are plotted one below the other. Thus, the points which are adjacent on any vertical line are exactly 24 hours apart. Rest and activity are randomly distributed throughout the day.
The Human Circadian Clock
Time isolation experiments analogous to those carried out with rodents have been performed with human volunteers. Subjects have lived for up to six months in special apartments without windows, telephones, television, radio or any contact with the outside world that might provide a clue as to the time of day. Under these conditions diurnal rhythms regulated by the circadian clock of sleep, activity, body temperature, hormone secretion and a range of other variables persist, but they no longer exhibit a period length of 24 hours. In the case of the typical human subject, the "free-running period" of the circadian system is greater than 24 hours, usually nearer to 25 hours.
Figure 5 presents a day-by-day plot of one man's pattern of sleep and wakefulness illustrating aspects of the circadian oscillation. The solid lines indicate sleep, the dotted lines indicate wakefulness, and the triangular symbol indicates the low point of the body temperature during each successive 24-hour period. The upper nine periods show sleep and wakefulness during a 9-day interval in a normal environment where the man was exposed to alternating sunlight and darkness, warmth and coldness, noise and quiet. These and many other factors in our environment that change rhythmically because of the earth's rotation "entrain" our endogenous circadian oscillation to a periodicity of exactly 24 hours. This is why we tend to go to bed and get up at about the same time each day.
Days four and five represent a typical weekend sleep pattern, with a later bedtime and arising time on two consecutive days. However, on the mornings after the later bedtimes it is difficult to continue sleeping until eight hours of sleep have been accumulated because the entrained circadian oscillation continues without interruption, and this means we are trying to sleep when the cycle is on the upswing. Note that the lowest body temperature occurs toward the end of sleep each night.
The middle section of the plot is very important. It illustrates what would happen if the individual were placed in a situation isolated from the normal environment and all the influences of the earth's rotation. In such an environment, temperature, noise levels, etc., would be constant. He would control the light himself, prepare his own meals when he was hungry, etc. There would be no clues (clocks, etc.) to tell him what time it was.
The seminal point is that the circadian oscillation in bodily functions would continue. It is innate, internal, built into the organism, and does not need to be "driven" from outside. The second important point is that the oscillation would no longer be exactly 24 hours. In an isolated environment, the individual would "free run." The latter term applies when the circadian rhythm is no longer entrained to 24 hours by environmental influences, but rather is oscillating at its own natural periodicity. The individual does things when he "feels like it." What is remarkable is that in such a case, the sleep-wakefulness rhythm of the body shows a periodicity that is close to 24 hours. In the case we have illustrated, the internal clock of the individual has a natural rhythm of exactly 25 hours. Thus, as he goes to bed when he becomes sleepy, his sleep starts about one hour later each night. During 24 days in isolation he drifts all the way around the clock until he is going to sleep at midnight again.
Although it would not be exactly 25 hours, the natural oscillation of most people would probably be from about twenty-four and a half hours to twenty-five and a half hours. This is why the term circadian is so appropriate to describe these natural oscillations. When our subject was "free running," the body temperature rhythm changed its phase relationship to sleep-wakefulness. Instead of reaching its low point toward the end of sleep, the low point occurred at the beginning of the sleep period. This change in the phase relationship between sleep and body temperature suggests that these two processes have distinct timing mechanisms.
The lower section of the figure illustrates the course of events when the individual leaves isolation and becomes re-entrained to a 24-hour day. Such re-entrainment is obviously easier when he has drifted all the way around the clock than if he emerged from isolation when he was going to bed at around noon. At any rate, as before the isolation experiment, the lights are turned off at midnight and turned on around 8 a.m. Meals are served at a certain time each day. Note that the bedtime becomes stable at around midnight and the low point of body temperature moves back toward the end of the sleep period.
Most humans, as we have indicated, have a natural periodicity of approximately 25 hours, which is easily entrained to the 24-hour day. However, in insomniacs and other sleep-disordered patients, something may be wrong with either the entrainment mechanism or the innate oscillation so that their bodies cannot oscillate in synchrony with their environment.
The 25 hour period of our internal clock accounts for two maladies which are common in the lives of almost everyone in our modern society. The first is the infamous "jet lag" which is a circadian disorder that results from changing time zones (and zeitgeber orientation) faster than the internal circadian clock can adjust.
Since the earliest days of commercial air travel, it has been recognized that jet lag is more severe, persistent and downright annoying after eastward travel than it is after westward travel (Figures 6 and 7).
This is due to the fact that the shift to later hours which occurs after a westward trip is in accord with the twenty-five hour circadian clock's tendency to drift to later hours. By contrast, when one travels eastward there is an advance to earlier hours, which is against the clock's natural tendency.
The effects of time displacement on the phase of the circadian rhythm of air pilots' perfomance in a simulator after simulated westward and eastward travel. Restoration of baseline performance rhythms takes longer after eastward displacement.
Flight 1: Travel in the same time zone does not produce any jet lag, even on a long 12-hour flight from Lima, Peru, to New York. Flight 2: Crossing 5 time zones on an 8-hour flight from Paris to New York causes moderate jet lag. Most people will take two-to three days to adjust. Flight 3: Crossing 11 time zones on a 10-hour flight from Copenhagen to Alaska would cause extreme jet lag. Most people require a minimum of a week to adjust.
The second common circadian disorder has been termed the "Monday Morning Blues" An individual who sleeps late on Saturday morning misses the normal early morning zeitgeber signal and as a consequence the 25-hour internal clock drifts approximately one hour later. The process is repeated on Sunday morning so that when the alarm clock rings at 6 a.m. on Monday morning, the body's clock is 2 hours behind and one has to struggle to get out of bed because it is 4 a.m. according to the body's internal clock.
The regular daily alternation between sleep and wakefulness, with the associated changes in neural and physiologic functions, is arguably the most fundamental of mammalian diurnal rhythms. Indeed, much of what appears to be circadian rhythmicity in physiological function is more precisely characterized as a secondary response to the rhythmic changes in sleep and waking states rather than to a direct influence of the circadian clock
The rhythmic nature of sleep has led some researchers to speculate that its principal function (or, more directly, the function of the sleepiness that accumulates upon its delay) is to rhythmically drive the organism to seek safety in inactivity deep in a burrow, nest or bed when the external environment is most hostile or least accommodating. While such a restrictive definition of the function of sleep would seem to belie the internal complexities of sleep and sleep states, it is nonetheless important to recognize that sleep and wakefulness are inextricably linked to the circadian clock that so strongly modulates their expression (see Part D., Functions of Sleep).
The consolidation of human sleep into a single major episode (give or take the occasional nap during lectures) makes it difficult to recognize the influence of the circadian clock under basal conditions. We begin to feel sleepy towards the end of the day and gravitate towards bed at 11 P.M. or midnight feeling that our sleepiness is more the result of a long day's activity than of the tolling of our internal clock. Given sufficient stimulation, most people have little trouble delaying bedtime by one or two hours. These minor imprecisions in the day to day timing of sleep onset and waking onset often seem more impressive than the marked consistency of overall sleep timing. Clearer evidence for the influence of circadian timing on basal sleep occurs when one examines the sleep of rodents. Instead of a single consolidated sleep episode, rodents such as the mouse sleep in shorter "bouts" of 5-30 minutes, which alternate with similar episodes of wakefulness. The circadian pattern arises from a systematic variation in the length of these sleep bouts. When isolated in complete darkness from all zeitgebers, rodents continue to exhibit regular circadian rhythms of sleep and wakefulness, typically with period lengths of slightly less than 24 hours. A good example of the circadian influence on human sleep are sleep deprivation experiments in which alertness increases in the morning after subjects have been up all night, even though they have not slept at all! Similarly, as noted above, humans living in isolation with no cues as to the time of day exhibit circadian rhythms of sleep and wakefulness with a period that is greater than 24 hours. Clearly, sleep results from the complex interaction of circadian and non-circadian influences.
Neither the basal condition nor the free-running condition establish that the circadian clock is directly controlling sleep and wakefulness. An alternative, and equally plausible hypothesis, is that the circadian clock controls a homeostatic mechanism in which sleepiness increases in proportion to the time awake and eventually results in sleep onset. The reverse homeostatic mechanism, in which alertness increases with the amount of time spent asleep, could account for the timing of the onset of wakefulness. Determination of the relative contribution of each mechanism to the timing of sleep requires dissection of their normally coincident influences. In one experiment in which this was done, subjects were put in bed and allowed to sleep for only one hour out of every 3 hours. This cycle of one hour in bed and two hours awake was maintained for several days around the clock. Thus, a total of 8 hours were spent in bed per day, but the hours were evenly distributed throughout each 24 hour period. Despite severe sleep deprivation accumulating over several days, subjects were still unable to sleep during the hour periods in the afternoon and evening that coincided with the peak of the circadian cycle when they would "normally" be awake.
Nonetheless, homeostatic influences on the timing of sleep are also important. After lesions of the suprachiasmatic nuclei have been made in animals, sleep and wakefulness are no longer rhythmic; alternating episodes of sleep and wakefulness which vary in length occur randomly throughout the day and night. However, if the animal is kept awake for a significant length of time, and then is allowed to fall asleep, the duration of this " recovery" sleep is increased above the normal baseline duration, thus supporting a role for homeostatic regulation of sleep and wakefulness.
The complex interaction between homeostatic and circadian influences in the timing of sleep is thought to explain the phenomenon of internal desynchronization" seen in humans during the free-running conditioning (i.e., what happens in the absence of zeitgebers). After varying lengths of exposure to free-running conditions, subjects will spontaneously exhibit long or short sleep-wake cycles with periods of 30-40 or 5-20 hours. The circadian clock maintains its normal near-24 hour period as evidenced by persistent rhythms in such parameters as body temperature (Figure 8). The mechanism of this breakdown in synchrony between sleep-waking and other circadian rhythms is unclear. It has been suggested that the long sleep-wake rhythm is the manifestation of a second internal clock with a longer and more variable period length. Under normal conditions the two oscillators are synchronized, but in internal desynchrony the sleep-wake oscillator breaks free. The anatomical location of this proposed second clock is not known.
This is a somewhat idealized graph of a person's body temperature, showing its more or less sinusoidal fluctuation throughout the day and night. Note that the maximum temperature occurs during the day and the minimum at night. This is a very good example of the circadian oscillation that is present in nearly all bodily functions. In the "old" days, people thought that the low nocturnal body temperature was a passive consequence of the general absence of muscle activity during sleep. However, as shown in the figure, the daily temperature cycle continues even if we do not sleep. The intrinsic circadian rhythms of bodily functions are not dependent upon the daily alternation of sleep-wakefulness. Rather, the circadian fluctuations influence the tendency to sleep or remain awake. The jet lag syndrome suffered by world travelers clearly illustrates this principle. Such travelers must change their clocks but cannot immediately change the smooth flow of their circadian rhythms, and, therefore, the trough of the oscillation occurs during the day, which makes them very drowsy, and the peak occurs at night, which makes their sleep fitful and restless.
It has also been
suggested that the long sleep-wake cycles are the result of variable
insensitivity to the homeostatic sleep drive. In this model, the cumulative
effects of 16-18 hours of wakefulness do not provide a signal sufficient in
amplitude to initiate sleep behavior, and the subject remains awake through the
trough of the next circadian cycle. Then, as sleepiness accumulates, the sleep
drive is reduced by the subsequent circadian peak and the subject elects to sleep
only on the falling phase of the next circadian cycle. Amazingly, subjects
experiencing internal desynchronization are subjectively unaware of the
extraordinary length of their sleep-wake cycles. At the end of 30 or more hours
of continuous wakefulness they report levels of subjective sleepiness and fatigue
that are indistinguishable from their normal reports of sleepiness at the end of
a conventional day (Figure 9).
Test subjects spent seventy-two hours without sleep and rated their fatigue every three hours on a scale in comparison with their normal fatigue (=100 percent). The feeling of fatigue was always highest in the early hours of the morning and lowest in the afternoon.
CIRCADIAN MODULATION OF SLEEP STATES
In addition to the control of overt sleep behavior, the internal
circadian clock controls the timing of sleep states as well. In the case of REM
sleep, circadian modulation of the expression of REM is relatively independent of
the timing of sleep itself. Thus, there is a circadian rhythm in REM propensity
with its peak near the minimum of body temperature, i.e., in the early morning
hours. Sleep that occurs coincident with this circadian peak in REM sleep has
relatively long REM periods, and if sleep onset occurs near the REM peak, the
latency from sleep onset to REM onset (the REM latency) will be very short. In
contrast, sleep that occurs in the late afternoon or evening, when the REM
propensity rhythm is at its minimum, exhibits brief REM periods and a long REM
latency. Normal nocturnal sleep spans the range from having a low to high REM
propensity, and this accounts for the distribution of REM periods; REM periods
increase in length across the night and REM occupies an increasing percentage of
total sleep time later in the night.
Sleepiness is modulated by the circadian clock as well as by the homeostatic
sleep drive and behavioral imperatives. The rhythmic variation is apparent both
when sleep is distributed throughout the day and under basal conditions with
normal consolidated nocturnal sleep. Under these conditions, sleep tendency
shows a nocturnal increase coincident with the trough in body temperature (Figure
8), but in addition there is a consistent peak in sleep tendency (a minimum in
sleep latency) in the early afternoon. A number of studies have demonstrated
that it persists independent of the timing of meals or other exogenous factors
that might augment sleepiness. This second peak in sleepiness corresponds with
the timing of most naps and siestas; thus it appears to reflect an endogenously
timed "nap" phase of the circadian cycle, the significance of which is
unclear.
One product of research on the circadian timing system in humans has been the
growing recognition of the role that disrupted circadian rhythmicity plays in the
generation of certain clinical sleep disorders. In addition to the jet-lag
syndrome and "the Monday Morning Blues", there are more serious chronic problems
for which principles of circadian physiology provide an understanding of both the
pathophysiology and potential treatments.
The Advanced Sleep Phase Syndrome, which occurs most often in elderly
individuals, is described in the following section entitled Circadian Rhythms and
Aging. In Delayed Sleep Phase Syndrome (DSPS), which usually occurs in adults,
the circadian system is shifted to a position markedly later than normal (Figure
10). The sleep propensity rhythm shifts with it so that the patient with DSPS
cannot fall asleep before 3 or 4 a.m., and they cannot wake up before noon
without extraordinary effort. An apparent defect in the circadian entrainment
mechanism prevents normal corrective shifts to earlier hours while allowing
stable entrainment to the 24 hour cycle. This locks patients into a kind of
permanent "jet-lag" and their functioning in a normal diurnal world is
difficult.
DISORDERS OF CIRCADIAN TIMING
A schematic illustration of a conventional sleep-waking schedule and the sleep-waking schedule of a delayed sleep phase syndrome and an advanced sleep phase syndrome. The timing of the preferred sleep period in each syndrome is designated with bold lines and dashed lines indicate the sleep problems when a conventional schedule is maintained.
Current treatment of DSPS exploits the
greater than 24 hour endogenous period of the internal clock by shifting the
phase progressively later around the clock until the desired orientation is
achieved. Experimental therapies using artificial bright light are based upon
the hypothesis that the defect in DSPS is due to a relative inadequacy of
zeitgebers. Controlled exposure to bright light in these patients during morning
hours provides for a phase advance which is not possible under standard
conditions. However, it should be noted that the efficacy of this treatment has
not yet been fully assessed.
Shift-work syndrome is not really a
disease, in that most patients who complain of this problem probably possess
perfectly normal circadian timing systems. Instead, the root of this problem
lies with the pressures of modern society which have dictated that an increasing
percentage of the work force must work nights either full time or as part of a
rotating shift schedule. Night work presents obvious problems for the circadian
clock, given that the available zeitgeber signals continue to enforce a
daytime orientation. Many shiftworkers do not adapt, and the conflict between
their work schedule and circadian orientation produces insomnia during the day
when they try to sleep and excessive sleepiness at night when they are trying to
work. Over time the resultant chronic sleep deprivation produces general stress
and a host of secondary medical disorders. Research on the importance of light
to entrainment of human circadian clocks may well help resolve this problem.
Entrainment failure is the persistence of a "free-running" sleep-wake rhythm
despite the presence of adequate zeitgeber signals in the environment.
This produces a complaint of cyclical insomnia in which patients report that they
sleep fine when the circadian system is in synchrony with the external world.
But as the clock drifts out of phase it prevents sleep during normal nocturnal
hours, resulting in insomnia lasting for several days until the clock drifts back
into a normal orientation and the cycle starts all over again. As might be
predicted, entrainment failure is much more common among the blind than among
those with normal eyesight. Cases have been reported in sighted individuals; the
mechanism of zeitgeber insensitivity in this population is not known.
Clock failure is a rare condition in otherwise normal individuals, but it has
been recognized for several years as one manifestation of a neurologic disease
involving the anterior hypothalamus. Thus, several cases have been described in
which episodes of sleep and wakefulness are scattered throughout the 24 hour
period in a random fashion analogous to that seen in animals with lesions of
their SCN (Figure 3). In each such case, a tumor or other destructive
neurological lesion has been identified as the cause.
Several studies of a variety of circadian variables have described a change in
circadian organization with advancing age. These changes are 1) a decrease in
free-running period; 2) a shift of phase to earlier hours; and 3) a decrease in
the amplitude of the circadian variation. Other studies have documented a loss
of neurons in the SCN with increasing age. It has been suggested that this
deterioration may underlie the changes in functional circadian organization due
to aging of the circadian clock. The advance of the sleep-wake cycle to earlier
hours produces an Advanced Sleep Phase Syndrome (ASPS). In ASPS elderly patients
report overwhelming sleepiness at 7 or 8 p.m. which makes it impossible to remain
awake. At 3 or 4 a.m. they awake and are unable to continue sleeping. However,
in contrast to DSPS, ASPS is largely compatible with societal demands imposed by
work or other responsibilities and so few older patients regard this as much of a
problem.
CIRCADIAN RHYTHMS AND AGING
| Proceed to Part H. |
| Return to Syllabus Home. |