Rapid suppression of bone formation marker in response to sleep restriction and circadian disruption in men.

We describe the time course of bone formation marker (P1NP) decline in men exposed to ~ 3 weeks of sleep restriction with concurrent circadian disruption. P1NP declined within 10 days and remained lower with ongoing exposure. These data suggest even brief exposure to sleep and circadian disruptions may disrupt bone metabolism. INTRODUCTION: A serum bone formation marker (procollagen type 1 N-terminal, P1NP) was lower after ~ 3 weeks of sleep restriction combined with circadian disruption. We now describe the time course of decline. METHODS: The ~ 3-week protocol included two segments: "baseline," ≥ 10-h sleep opportunity/day × 5 days; "forced desynchrony" (FD), recurring 28 h day (circadian disruption) with sleep restriction (~ 5.6-h sleep per 24 h). Fasted plasma P1NP was measured throughout the protocol in nine men (20-59 years old). We tested the hypothesis that PINP would steadily decline across the FD intervention because the magnitude of sleep loss and circadian misalignment accrued as the protocol progressed. A piecewise linear regression model was used to estimate the slope (ß) as ΔP1NP per 24 h with a change point mid-protocol to estimate the initial vs. prolonged effects of FD exposure. RESULTS: Plasma P1NP levels declined significantly within the first 10 days of FD ([Formula: see text] = - 1.33 µg/L per 24 h, p < 0.0001) and remained lower than baseline with prolonged exposure out to 3 weeks ([Formula: see text] = - 0.18 µg/L per 24 h, p = 0.67). As previously reported, levels of a bone resorption marker (C-telopeptide (CTX)) were unchanged. CONCLUSION: Sleep restriction with concurrent circadian disruption induced a relatively rapid decline in P1NP (despite no change in CTX) and levels remained lower with ongoing exposure. These data suggest (1) even brief sleep restriction and circadian disruption can adversely affect bone metabolism, and (2) there is no P1NP recovery with ongoing exposure that, taken together, could lead to lower bone density over time.

24-hour profile of serum sclerostin and its association with bone biomarkers in men.

The osteocyte's role in orchestrating diurnal variations in bone turnover markers (BTMs) is unclear. We identified no rhythm in serum sclerostin (osteocyte protein). These results suggest that serum sclerostin can be measured at any time of day and the osteocyte does not direct the rhythmicity of other BTMs in men. INTRODUCTION: The osteocyte exerts important effects on bone remodeling, but its rhythmicity and effect on the rhythms of other bone cells are not fully characterized. The purpose of this study was to determine if serum sclerostin displays rhythmicity over a 24-h interval, similar to that of other bone biomarkers. METHODS: Serum sclerostin, FGF-23, CTX, and P1NP were measured every 2 h over a 24-h interval in ten healthy men aged 20-65 years. Maximum likelihood estimates of the parameters in a repeated measures model were used to determine if these biomarkers displayed a diurnal, sinusoidal rhythm. RESULTS: No discernible 24-h rhythm was identified for sclerostin (p = 0.99) or P1NP (p = 0.65). CTX rhythmicity was confirmed (p < 0.001), peaking at 05:30 (range 01:30-07:30). FGF-23 levels were also rhythmic (p < 0.001), but time of peak was variable (range 02:30-11:30). The only significant association identified between these four bone biomarkers was for CTX and P1NP mean 24-h metabolite levels (r = 0.65, p = 0.04). CONCLUSIONS: Sclerostin levels do not appear to be rhythmic in men. This suggests that in contrast to CTX, serum sclerostin could be measured at any time of day. The 24-h profiles of FGF-23 suggest that a component of osteocyte function is rhythmic, but its timing is variable. Our results do not support the hypothesis that osteocytes direct the rhythmicity of other bone turnover markers (CTX), at least not via a sclerostin-mediated mechanism.

Use of transdermal melatonin delivery to improve sleep maintenance during daytime.

Oral melatonin (MEL) can improve daytime sleep, but the hormone's short elimination half-life limits its use as a hypnotic in shift workers and individuals with jet lag or other sleep problems. Here we show, in healthy subjects, that transdermal delivery of MEL during the daytime can elevate plasma MEL and reduce waking after sleep onset, by promoting sleep in the latter part of an 8-h sleep opportunity. Transdermal MEL may have advantages over fast-release oral MEL in improving sleep maintenance during adverse circadian phases.

Sleep and circadian rhythms in humans.

During the past 50 years, converging evidence reveals that the fundamental properties of the human circadian system are shared in common with those of other organisms. Concurrent data from multiple physiological rhythms in humans revealed that under some conditions, rhythms oscillated at different periods within the same individuals and led to the conclusion 30 years ago that the human circadian system was composed of multiple oscillators organized hierarchically; this inference has recently been confirmed using molecular techniques in species ranging from unicellular marine organisms to mammals. Although humans were once thought to be insensitive to the resetting effects of light, light is now recognized as the principal circadian synchronizer in humans, capable of eliciting weak (Type 1) or strong (Type 0) resetting, depending on stimulus strength and timing. Realization that circadian photoreception could be maintained in the absence of sight was first recognized in blind humans, as was the property of adaptation of the sensitivity of circadian photoreception to prior light history. In sighted humans, the intrinsic circadian period is very tightly distributed around approximately 24.2 hours and exhibits aftereffects of prior entrainment. Phase angle of entrainment is dependent on circadian period, at least in young adults. Circadian pacemakers in humans drive daily variations in many physiologic and behavioral variables, including circadian rhythms in alertness and sleep propensity. Under entrained conditions, these rhythms interact with homeostatic regulation of the sleep/wake cycle to determine the ability to sustain vigilance during the day and to sleep at night. Quantitative understanding of the fundamental properties of the multioscillator circadian system in humans and their interaction with sleep/wake homeostasis has many applications to health and disease, including the development of treatments for circadian rhythm and sleep disorders.

Reduced sleep efficiency in cervical spinal cord injury; association with abolished night time melatonin secretion.

STUDY DESIGN: Case-controlled preliminary observational study. OBJECTIVE: Melatonin is usually secreted only at night and may influence sleep. We previously found that complete cervical spinal cord injury (SCI) interrupts the neural pathway required for melatonin secretion. Thus, we investigated whether the absence of night time melatonin in cervical SCI leads to sleep disturbances. SETTING: General Clinical Research Center, Brigham and Women's Hospital, Boston, USA. METHODS: In an ancillary analysis of data collected in a prior study, we assessed the sleep patterns of three subjects with cervical SCI plus absence of nocturnal melatonin (SCI levels: C4A, C6A, C6/7A) and two control patients with thoracic SCI plus normal melatonin rhythms (SCI levels: T4A, T5A). We also compared those results to the sleep patterns of 10 healthy control subjects. RESULTS: The subjects with cervical SCI had significantly lower sleep efficiency (median 83%) than the control subjects with thoracic SCI (93%). The sleep efficiency of subjects with thoracic SCI was not different from that of healthy control subjects (94%). There was no difference in the proportion of the different sleep stages, although there was a significantly increased REM-onset latency in subjects with cervical SCI (220 min) as compared to subjects with thoracic SCI (34 min). The diminished sleep in cervical SCI was not associated with sleep apnea or medication use. CONCLUSION: We found that cervical SCI is associated with decreased sleep quality. A larger study is required to confirm these findings. If confirmed, the absence of night time melatonin in cervical SCI may help explain their sleep disturbances, raising the possibility that melatonin replacement therapy could help normalize sleep in this group.

Photic resetting of the human circadian pacemaker in the absence of conscious vision.

Ocular light exposure patterns are the primary stimuli for entraining the human circadian system to the local 24-h day. Many totally blind persons cannot use these stimuli and, therefore, have circadian rhythms that are not entrained. However, a few otherwise totally blind persons retain the ability to suppress plasma melatonin concentrations after ocular light exposure, probably using a neural pathway that includes the site of the human circadian pacemaker, suggesting that light information is reaching this site. To test definitively whether ocular light exposure could affect the circadian pacemaker of some blind persons and whether melatonin suppression in response to bright light correlates with light-induced phase shifts of thecircadian system, the authorsperformed experiments with 5 totally blind volunteers using a protocol known to induce phase shifts of the circadian pacemaker in sighted individuals. In the 2 blind individuals who maintained light-induced melatonin suppression, the circadian system was shifted by appropriately timed bright-light stimuli. These data demonstrate that light can affect the circadian pacemaker of some totally blind individuals--either by altering the phase of the circadian pacemaker or by affecting its amplitude. They are consistent with data from animal studies demonstrating that there are different neural pathways and retinal cells that relay photic information to the brain: one for conscious light perception and the other for non-image-forming functions.

Separation of circadian and wake duration-dependent modulation of EEG activation during wakefulness.

The separate contribution of circadian rhythmicity and elapsed time awake on electroencephalographic (EEG) activity during wakefulness was assessed. Seven men lived in an environmental scheduling facility for 4 weeks and completed fourteen 42.85-h 'days', each consisting of an extended (28.57-h) wake episode and a 14.28-h sleep opportunity. The circadian rhythm of plasma melatonin desynchronized from the 42.85-h day. This allowed quantification of the separate contribution of circadian phase and elapsed time awake to variation in EEG power spectra (1-32 Hz). EEG activity during standardized behavioral conditions was markedly affected by both circadian phase and elapsed time awake in an EEG frequency- and derivation-specific manner. The nadir of the circadian rhythm in alpha (8-12 Hz) activity in both fronto-central and occipito-parietal derivations occurred during the biological night, close to the crest of the melatonin rhythm. The nadir of the circadian rhythm of theta (4.5-8 Hz) and beta (20-32 Hz) activity in the fronto-central derivation was located close to the onset of melatonin secretion, i.e. during the wake maintenance zone. As time awake progressed, delta frequency (1-4.5 Hz) and beta (20-32 Hz) activity rose monotonically in frontal derivations. The interaction between the circadian and wake-dependent increase in frontal delta was such that the intrusion of delta was minimal when sustained wakefulness coincided with the biological day, but pronounced during the biological night. Our data imply that the circadian pacemaker facilitates frontal EEG activation during the wake maintenance zone, by generating an arousal signal that prevents the intrusion of low-frequency EEG components, the propensity for which increases progressively during wakefulness.

Intrinsic near-24-h pacemaker period determines limits of circadian entrainment to a weak synchronizer in humans.

Endogenous circadian clocks are robust regulators of physiology and behavior. Synchronization or entrainment of biological clocks to environmental time is adaptive and important for physiological homeostasis and for the proper timing of species-specific behaviors. We studied subjects in the laboratory for up to 55 days each to determine the ability to entrain the human clock to a weak circadian synchronizing stimulus [scheduled activity-rest cycle in very dim (approximately 1.5 lux in the angle of gaze) light-dark cycle] at three approximately 24-h periods: 23.5, 24.0, and 24.6 h. These studies allowed us to test two competing hypotheses as to whether the period of the human circadian pacemaker is near to or much longer than 24 h. We report here that imposition of a sleep-wake schedule with exposure to the equivalent of candle light during wakefulness and darkness during sleep is usually sufficient to maintain circadian entrainment to the 24-h day but not to a 23.5- or 24.6-h day. Our results demonstrate functionally that, in normally entrained sighted adults, the average intrinsic circadian period of the human biological clock is very close to 24 h. Either exposure to very dim light and/or the scheduled sleep-wake cycle itself can entrain this near-24-h intrinsic period of the human circadian pacemaker to the 24-h day.

Sleep, performance, circadian rhythms, and light-dark cycles during two space shuttle flights.

Sleep, circadian rhythm, and neurobehavioral performance measures were obtained in five astronauts before, during, and after 16-day or 10-day space missions. In space, scheduled rest-activity cycles were 20-35 min shorter than 24 h. Light-dark cycles were highly variable on the flight deck, and daytime illuminances in other compartments of the spacecraft were very low (5.0-79.4 lx). In space, the amplitude of the body temperature rhythm was reduced and the circadian rhythm of urinary cortisol appeared misaligned relative to the imposed non-24-h sleep-wake schedule. Neurobehavioral performance decrements were observed. Sleep duration, assessed by questionnaires and actigraphy, was only approximately 6.5 h/day. Subjective sleep quality diminished. Polysomnography revealed more wakefulness and less slow-wave sleep during the final third of sleep episodes. Administration of melatonin (0.3 mg) on alternate nights did not improve sleep. After return to earth, rapid eye movement (REM) sleep was markedly increased. Crewmembers on these flights experienced circadian rhythm disturbances, sleep loss, decrements in neurobehavioral performance, and postflight changes in REM sleep.

Association of intrinsic circadian period with morningness-eveningness, usual wake time, and circadian phase.

The biological basis of preferences for morning or evening activity patterns ("early birds" and "night owls") has been hypothesized but has remained elusive. The authors reported that, compared with evening types, the circadian pacemaker of morning types was entrained to an earlier hour with respect to both clock time and wake time. The present study explores a chronobiological mechanism by which the biological clock of morning types may be set to an earlier hour. Intrinsic period, a fundamental property of the circadian system, was measured in a month-long inpatient study. A subset of participants also had their circadian phase assessed. Participants completed a morningness-eveningness questionnaire before study. Circadian period was correlated with morningness-eveningness, circadian phase, and wake time, demonstrating that a fundamental property of the circadian pacemaker is correlated with the behavioral trait of morningness-eveningness.

Microgravity reduces sleep-disordered breathing in humans.

To understand the factors that alter sleep quality in space, we studied the effect of spaceflight on sleep-disordered breathing. We analyzed 77 8-h, full polysomnographic recordings (PSGs) from five healthy subjects before spaceflight, on four occasions per subject during either a 16- or 9-d space shuttle mission and shortly after return to earth. Microgravity was associated with a 55% reduction in the apnea-hypopnea index (AHI), which decreased from a preflight value of 8.3 +/- 1.6 to 3.4 +/- 0.8 events/h inflight. This reduction in AHI was accompanied by a virtual elimination of snoring, which fell from 16.5 +/- 3.0% of total sleep time preflight to 0.7 +/- 0.5% inflight. Electroencephalogram (EEG) arousals also decreased in microgravity (by 19%), and this decrease was almost entirely a consequence of the reduction in respiratory-related arousals, which fell from 5.5 +/- 1.2 arousals/h preflight to 1.8 +/- 0.6 inflight. Postflight there was a return to near or slightly above preflight levels in these variables. We conclude that sleep quality during spaceflight is not degraded by sleep-disordered breathing. This is the first direct demonstration that gravity plays a dominant role in the generation of apneas, hypopneas, and snoring in healthy subjects.

Age-related increase in awakenings: impaired consolidation of nonREM sleep at all circadian phases.

STUDY OBJECTIVES: (1) To assess the circadian and sleep-dependent regulation of the frequency and duration of awakenings in young and older people; (2) to determine whether age-related deterioration of sleep consolidation is related to an increase in the frequency or duration of awakenings; (3) to determine whether pre-awakening sleep structure is preferentially enriched by REM sleep or nonREM sleep and (4) to determine whether sleep structure prior to awakenings is affected by age. DESIGN: Between age-group comparison of sleep consolidation and sleep structure preceding awakenings. SETTING: Environmental Scheduling Facility, General Clinical Research Center. PARTICIPANTS: Eleven healthy young men (21-30 years) and 13 older healthy men (n=9) and women (n=4) (64-74 years). INTERVENTIONS: Forced desynchrony between the sleep-wake cycle and circadian rhythms by scheduling of the rest-activity cycle to 28-h for 21-25 cycles. MEASUREMENTS AND RESULTS: Circadian and sleep-dependent regulation of the frequency and duration of awakenings and of sleep structure preceding awakenings were assessed in 482 sleep episodes (9h 20 min each). The circadian modulation of wakefulness within sleep episodes was primarily related to a variation in the duration of awakenings. In contrast, the age-related reduction of sleep consolidation was primarily related to an increase in the frequency of awakenings. Whereas in both young and older subjects pre-awakening sleep contained more REM sleep than overall sleep, this enrichment of REM sleep (i.e., the gating of wakefulness by REM sleep) was diminished in older people. In older people, preawakening sleep contained more nonREM sleep and stage two sleep in particular, than in young people. CONCLUSIONS: At all circadian phases, the age-related reduction of sleep consolidation is primarily related to a reduction in the consolidation of nonREM sleep.

Absence of an increase in the duration of the circadian melatonin secretory episode in totally blind human subjects.

The daily rhythm of melatonin influences multiple physiological measures, including sleep tendency, circadian rhythms, and reproductive function in seasonally breeding mammals. The biological signal for photoperiodic changes in seasonally breeding mammals is a change in the duration of melatonin secretion, which in a natural environment reflects the different durations of daylight across the year, with longer nights leading to a longer duration of melatonin secretion. These seasonal changes in the duration of melatonin secretion do not simply reflect the known acute suppression of melatonin secretion by ocular light exposure, but also represent long-term changes in the endogenous nocturnal melatonin episode that persist in constant conditions. As the eyes of totally blind individuals do not transmit ocular light information, we hypothesized that the duration of the melatonin secretory episode in blind subjects would be longer than those in sighted individuals, who are exposed to light for all their waking hours in an urban environment. We assessed the melatonin secretory profile during constant posture, dim light conditions in 17 blind and 157 sighted adults, all of whom were healthy and using no prescription or nonprescription medications. The duration of melatonin secretion was not significantly different between blind and sighted individuals. Healthy blind individuals after years without ocular light exposure do not have a longer duration of melatonin secretion than healthy sighted individuals.

Efficacy of bright light and sleep/darkness scheduling in alleviating circadian maladaptation to night work.

We tested the hypothesis that circadian adaptation to night work is best achieved by combining bright light during the night shift and scheduled sleep in darkness. Fifty-four subjects participated in a shift work simulation of 4 day and 3 night shifts followed by a 38-h constant routine (CR). Subjects received 2,500 lux (Bright Light) or 150 lux (Room Light) during night shifts and were scheduled to sleep (at home in darkened bedrooms) from 0800 to 1600 (Fixed Sleep) or ad libitum (Free Sleep). Dim light melatonin onset (DLMO) was measured before and after the night shifts. Both Fixed Sleep and Bright Light conditions significantly phase delayed DLMO. Treatments combined additively, with light leading to larger phase shifts. Free Sleep subjects who spontaneously adopted consistent sleep schedules adapted better than those who did not. Neither properly timed bright light nor fixed sleep schedules were consistently sufficient to shift the melatonin rhythm completely into the sleep episode. Scheduling of sleep/darkness should play a major role in prescriptions for overcoming shift work-related phase misalignment.

Circadian phase resetting in older people by ocular bright light exposure.

BACKGROUND: Aging is associated with frequent complaints about earlier bedtimes and waketimes. These changes in sleep timing are associated with an earlier timing of multiple endogenous rhythms, including core body temperature (CBT) and plasma melatonin, driven by the circadian pacemaker. One possible cause of the age-related shift of endogenous circadian rhythms and the timing of sleep relative to clock time is a change in the phase-shifting capacity of the circadian pacemaker in response to the environmental light-dark cycle, the principal synchronizer of the human circadian system. METHODS: We studied the response of the circadian system of 24 older men and women and 23 young men to scheduled exposure to ocular bright light stimuli. Light stimuli were 5 hours in duration, administered for 3 consecutive days at an illuminance of approximately 10,000 lux. Light stimuli were scheduled 1.5 or 3.5 hours after the CBT nadir to induce shifts of endogenous circadian pacemaker to an earlier hour (phase advances) or were scheduled 1.5 hours before the CBT nadir to induce shifts to a later hour (phase delays). The rhythms of CBT and plasma melatonin assessed under constant conditions served as markers of circadian phase. RESULTS: Bright light stimuli elicited robust responses of the circadian timing system in older people; both phase advances and phase delays were induced. The magnitude of the phase delays did not differ significantly between older and younger individuals, but the phase advances were significantly attenuated in older people. CONCLUSIONS: The attenuated response to light stimuli that induce phase advances does not explain the advanced phase of the circadian pacemaker in older people. The maintained responsiveness of the circadian pacemaker to light implies that scheduled bright light exposure can be used to treat circadian phase disturbances in older people.

The timing of the human circadian clock is accurately represented by the core body temperature rhythm following phase shifts to a three-cycle light stimulus near the critical zone.

A double-stimulus experiment was conducted to evaluate the phase of the underlying circadian clock following light-induced phase shifts of the human circadian system. Circadian phase was assayed by constant routine from the rhythm in core body temperature before and after a three-cycle bright-light stimulus applied near the estimated minimum of the core body temperature rhythm. An identical, consecutive three-cycle light stimulus was then applied, and phase was reassessed. Phase shifts to these consecutive stimuli were no different from those obtained in a previous study following light stimuli applied under steady-state conditions over a range of circadian phases similar to those at which the consecutive stimuli were applied. These data suggest that circadian phase shifts of the core body temperature rhythm in response to a three-cycle stimulus occur within 24 h following the end of the 3-day light stimulus and that this poststimulus temperature rhythm accurately reflects the timing of the underlying circadian clock.

Dynamic resetting of the human circadian pacemaker by intermittent bright light.

In humans, experimental studies of circadian resetting typically have been limited to lengthy episodes of exposure to continuous bright light. To evaluate the time course of the human endogenous circadian pacemaker's resetting response to brief episodes of intermittent bright light, we studied 16 subjects assigned to one of two intermittent lighting conditions in which the subjects were presented with intermittent episodes of bright-light exposure at 25- or 90-min intervals. The effective duration of bright-light exposure was 31% or 63% compared with a continuous 5-h bright-light stimulus. Exposure to intermittent bright light elicited almost as great a resetting response compared with 5 h of continuous bright light. We conclude that exposure to intermittent bright light produces robust phase shifts of the endogenous circadian pacemaker. Furthermore, these results demonstrate that humans, like other species, exhibit an enhanced sensitivity to the initial minutes of bright-light exposure.

Dose-response relationship for light intensity and ocular and electroencephalographic correlates of human alertness.

Light can elicit both circadian and acute physiological responses in humans. In a dose response protocol men and women were exposed to illuminances ranging from 3 to 9100 lux for 6.5 h during the early biological night after they had been exposed to <3 lux for several hours. Light exerted an acute alerting response as assessed by a reduction in the incidence of slow-eye movements, a reduction of EEG activity in the theta-alpha frequencies (power density in the 5-9 Hz range) as well as a reduction in self-reported sleepiness. This alerting response was positively correlated with the degree of melatonin suppression by light. In accordance with the dose response function for circadian resetting and melatonin suppression, the responses of all three indices of alertness to variations in illuminance were consistent with a logistic dose response curve. Half of the maximum alerting response to bright light of 9100 lux was obtained with room light of approximately 100 lux. This sensitivity to light indicates that variations in illuminance within the range of typical, ambient, room light (90-180 lux) can have a significant impact on subjective alertness and its electrophysiologic concomitants in humans during the early biological night.

Physiological effects of light on the human circadian pacemaker.

The physiology of the human circadian pacemaker and its influence and on the daily organization of sleep, endocrine and behavioral processes is an emerging interest in science and medicine. Understanding the development, organization and fundamental properties underlying the circadian timing system may provide insight for the application of circadian principles to the practice of clinical medicine, both diagnostically (interpretation of certain clinical tests are dependent on time of day) and therapeutically (certain pharmacological responses vary with the time of day). The light-dark cycle is the most powerful external influence acting upon the human circadian pacemaker. It has been shown that timed exposure to light can both synchronize and reset the phase of the circadian pacemaker in a predictable manner. The emergence of detectable circadian rhythmicity in the neonatal period is under investigation (as described elsewhere in this issue). Therefore, the pattern of light exposure provided in the neonatal intensive care setting has implications. One recent study identified differences in both amount of sleep time and weight gain in infants maintained in a neonatal intensive care environment that controlled the light-dark cycle. Unfortunately, neither circadian phase nor the time of day has been considered in most clinical investigations. Further studies with knowledge of principles characterizing the human circadian timing system, which governs a wide array of physiological processes, are required to integrate these findings with the practice of clinical medicine.