Identification of a Preliminary Plasma Metabolome-based Biomarker for Circadian Phase in Humans.

Measuring individual circadian phase is important to diagnose and treat circadian rhythm sleep-wake disorders and circadian misalignment, inform chronotherapy, and advance circadian science. Initial findings using blood transcriptomics to predict the circadian phase marker dim-light melatonin onset (DLMO) show promise. Alternatively, there are limited attempts using metabolomics to predict DLMO and no known omics-based biomarkers predict dim-light melatonin offset (DLMOff). We analyzed the human plasma metabolome during adequate and insufficient sleep to predict DLMO and DLMOff using one blood sample. Sixteen (8 male/8 female) healthy participants aged 22.4 ± 4.8 years (mean ± SD) completed an in-laboratory study with 3 baseline days (9 h sleep opportunity/night), followed by a randomized cross-over protocol with 9-h adequate sleep and 5-h insufficient sleep conditions, each lasting 5 days. Blood was collected hourly during the final 24 h of each condition to independently determine DLMO and DLMOff. Blood samples collected every 4 h were analyzed by untargeted metabolomics and were randomly split into training (68%) and test (32%) sets for biomarker analyses. DLMO and DLMOff biomarker models were developed using partial least squares regression in the training set followed by performance assessments using the test set. At baseline, the DLMOff model showed the highest performance (0.91 R2 and 1.1 ± 1.1 h median absolute error ± interquartile range [MdAE ± IQR]), with significantly (p < 0.01) lower prediction error versus the DLMO model. When all conditions (baseline, 9 h, and 5 h) were included in performance analyses, the DLMO (0.60 R2; 2.2 ± 2.8 h MdAE; 44% of the samples with an error under 2 h) and DLMOff (0.62 R2; 1.8 ± 2.6 h MdAE; 51% of the samples with an error under 2 h) models were not statistically different. These findings show promise for metabolomics-based biomarkers of circadian phase and highlight the need to test biomarkers that predict multiple circadian phase markers under different physiological conditions.

Sleep duration and bone health measures in older men.

The associations between objective measures of sleep duration and bone outcomes in older men are unknown. No consistent, significant association was identified between sleep duration and bone mineral density (BMD) in the current analysis. However, future research should determine if vitamin D status modifies this relationship. INTRODUCTION: Prior studies, predominantly in women, reported that long and short self-reported sleep duration are associated with lower BMD. Associations between actigraphy-determined sleep duration and BMD or bone turnover markers (BTMs) in older men are unknown. METHODS: Men in The Osteoporotic Fractures in Men (MrOS) Study with wrist actigraphy and concurrent BMD assessment but without comorbidities affecting bone health were included. Sleep duration was considered as a continuous (N = 1926) and dichotomized variable where men were classified as getting the recommended (7-8 h/night; N = 478) or short (< 6 h/night; N = 577) sleep. The cross-sectional association between BMD, BTMs, and sleep duration was examined using a t test or linear regression, where appropriate, in unadjusted and adjusted models. RESULTS: There were no clinically or statistically significant differences in BMD at the L-spine, total hip, or femoral neck between men getting the recommended vs. short sleep duration, using actigraphy or self-reported sleep duration (all p ≥ 0.07). When sleep duration was considered as a continuous variable, femoral neck BMD was higher in men with longer self-reported sleep duration (ß = 0.006 ±0.003, p = 0.02), but this was not significant after further adjustment. In men with low 25OHD (< 20 ng/mL), longer actigraphy-determined sleep duration was associated with higher total hip BMD (ß = 0.016 ± 0.008; p = 0.04). Sleep duration and BTMs were not associated. CONCLUSION: Sleep duration was not associated with hip or L-spine BMD or BTMs in older men. Future research should determine if vitamin D status or other factors modify this relationship.

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.

Association between objective sleep duration and bone mineral density in older postmenopausal women from the Study of Osteoporotic Fractures (SOF).

Methodological limitations preclude determination of the association between sleep duration and bone mineral density (BMD) from existing literature. This was the first study to use objective sleep duration to determine its association with BMD. Nocturnal sleep duration, assessed objectively (actigraphy) or subjectively (questionnaire), was not independently associated with BMD in postmenopausal women. INTRODUCTION: Both long and short self-reported sleep durations are associated with low bone mineral density (BMD) in men and women. The association between sleep duration measured by actigraphy and BMD in postmenopausal women is unknown. METHODS: The Study of Osteoporotic Fractures (SOF) ancillary sleep study was used to determine the association between sleep duration and BMD at the total hip and femoral neck in postmenopausal women ≥ 75 years old. Sleep duration was assessed by wrist actigraphy (average 4 nights) and questionnaire. BMD was compared between postmenopausal women with short (< 6 h/night) vs. NIH-recommended (7-8 h/night) sleep durations. Data were analyzed using a 2-sample t test (unadjusted) and multivariate regression model (adjusted). Simple linear regression was used to estimate the difference in BMD per additional hour of sleep when sleep duration was considered as a continuous, rather than dichotomized, variable. RESULTS: Total hip BMD was higher in women with actigraphically assessed shorter sleep duration in unadjusted models only. No clinically or statistically significant differences in total hip or femoral neck BMD were observed according to nocturnal sleep duration after adjusting for body mass index (BMI) in dichotomized (N = 874) or continuous (N = 1624) sleep duration models or when subjective sleep duration was used. When sleep duration included daytime naps, longer sleep duration was associated with lower total hip BMD (ß = - 0.005, p = 0.04). CONCLUSIONS: Nocturnal sleep duration, whether assessed objectively (actigraphy) or subjectively (questionnaire), was not independently associated with BMD in older postmenopausal women.

Role of sleep and circadian disruption on energy expenditure and in metabolic predisposition to human obesity and metabolic disease.

Weight gain, obesity and diabetes have reached alarming levels in the developed world. Traditional risk factors such as over-eating, poor nutritional choices and lack of exercise cannot fully account for the high prevalence of metabolic disease. This review paper examines the scientific evidence on two novel risk factors that contribute to dys-regulated metabolic physiology: sleep disruption and circadian misalignment. Specifically, fundamental relationships between energy metabolism and sleep and circadian rhythms and the impact of sleep and circadian disruption on metabolic physiology are examined. Millions of individuals worldwide do not obtain sufficient sleep for healthy metabolic function, and many participate in shift work and social activities at times when the internal physiological clock is promoting sleep. These behaviours predispose an individual for poor metabolic health by promoting excess caloric intake in response to reduced sleep, food intake at internal biological times when metabolic physiology is not prepared, decreased energy expenditure when wakefulness and sleep are initiated at incorrect internal biological times, and disrupted glucose metabolism during short sleep and circadian misalignment. In addition to the traditional risk factors of poor diet and exercise, disturbed sleep and circadian rhythms represent modifiable risk factors for prevention and treatment of metabolic disease and for promotion of healthy metabolism.

Impaired endogenous fibrinolytic capacity in prehypertensive men.

Prehypertension (blood pressure (BP) 120-139/80-89 mm Hg) is associated with an increased risk for future atherothrombotic events. Although the mechanisms underlying this elevated risk are not completely understood, one possibility is that prehypertension is associated with impaired endothelial fibrinolytic capacity. We tested the hypothesis that vascular endothelial release of tissue-type plasminogen activator (t-PA) is impaired in prehypertensive men. Net endothelial release of t-PA was determined, in vivo, in response to intrabrachial infusions of bradykinin (12.5, 25, 50 ng per 100 ml tissue per min) and sodium nitroprusside at (1.0, 2.0, 4.0 µg per 100 ml tissue per min) in 42 middle-age and older men: 16 normotensive (BP range: 100-119/57-79 mm Hg); 16 prehypertensive (BP range: 120-139/76-89 mm Hg); and 10 hypertensive (BP range: 140-150/74-100 mm Hg). Net release of t-PA antigen was ~25% lower (P<0.05) in the prehypertensive (-0.9 ± 0.8 to 42.4 ± 5.3 ng per 100 ml tissue per min) compared with the normotensive (0.5 ± 1.0 to 53.9 ± 6.5 ng per 100 ml tissue per min) men. There was no significant difference in t-PA release between the hypertensive (-1.8 ± 1.6 to 40.8 ± 6.6 ng per 100 ml tissue per min) and prehypertensive groups. Sodium nitroprusside did not significantly alter the t-PA release in any group. These data indicate that endothelial t-PA release is diminished in prehypertensive men. Further, the level of impairment in t-PA release seen with clinical hypertension is already apparent in the prehypertensive state. Impaired endothelial fibrinolytic function may underlie the increased atherothrombotic risk associated with BP in the prehypertensive range.

Repeated fear-induced diurnal rhythm disruptions predict PTSD-like sensitized physiological acute stress responses in F344 rats.

AIM: To identify objective factors that can predict future sensitized stress responses, thus allowing for effective intervention prior to developing sensitization and subsequent stress-related disorders, including post-traumatic stress disorder (PTSD). METHODS: Adult male F344 rats implanted with biotelemetry devices were exposed to repeated conditioned fear or control conditions for 22 days followed by exposure to either no, mild or severe acute stress on day 23. Diurnal rhythms of locomotor activity (LA), heart rate (HR) and core body temperature (CBT) were biotelemetrically monitored throughout the study. In a subset of rat not implanted, corticosterone and indices of chronic stress were measured immediately following stress. RESULTS: Rats exposed to repeated fear had fear-evoked increases in behavioural freezing and HR/CBT during exposure to the fear environment and displayed indices of chronic stress. Repeated fear produced flattening of diurnal rhythms in LA, HR and CBT. Repeated fear did not sensitize the corticosterone response to acute stress, but produced sensitized HR/CBT responses following acute stress, relative to the effect of acute stress in the absence of a history of repeated fear. Greater diurnal rhythm disruptions during repeated fear predicted sensitized acute stress-induced physiological responses. Rats exposed to repeated fear also displayed flattened diurnal LA and basal increases in HR. CONCLUSIONS: Exposure to repeated fear produces outcomes consistent with those observed in PTSD. The results suggest that diurnal rhythm disruptions during chronic stressors may help predict sensitized physiological stress responses following traumatic events. Monitoring diurnal disruptions during repeated stress may thus help predict susceptibility to PTSD.

Topical aminolaevulinic acid-photodynamic therapy produces an inflammatory infiltrate but reduces Langerhans cells in healthy human skin in vivo.

BACKGROUND: Topical photodynamic therapy (PDT) elicits a therapeutic response in both skin cancer and immune-mediated skin disorders. While PDT induces direct cell death, host inflammatory and immune responses to PDT may contribute to the therapeutic effects. OBJECTIVES: To examine the impact of topical PDT on leucocyte trafficking and mediators of chemotaxis in healthy human skin. METHODS: Aminolaevulinic acid (ALA)-PDT was performed on the buttock skin of seven healthy volunteers. Biopsies for immunohistochemical assessment were taken 1, 4 and 24 h post-PDT and from untreated contralateral buttock skin (baseline). RESULTS: A significant dermal neutrophilic infiltrate appeared early, peaking at 4 h (P < 0·01) and returning to near baseline by 24 h. Expression of E-selectin was significantly higher at 4 h (P < 0·05) and correlated strongly with neutrophil numbers (r = 0·93). Expression of intercellular adhesion molecule 1 was significantly elevated after 24 h (P < 0·05) with an apparent gradual increase in CD4+ T cells up to this time point. Notably, epidermal Langerhans cells were significantly reduced 24 h post-PDT compared with baseline (P < 0·01) and comprised a significantly larger proportion of cells with migratory rather than dendritic morphology (P < 0·05). The number of epidermal cells expressing tumour necrosis factor-α significantly increased at 4 h (P < 0·05) and remained elevated 24 h post-PDT, whereas no significant change in expression of interleukin (IL)-1ß or IL-8 was seen. CONCLUSIONS: Reduction of Langerhans cells by topical PDT of human skin may play a significant role in PDT-induced local immunosuppression, potentially benefiting the treatment of immune-mediated skin disorders but negatively impacting on antitumour responses. Further exploration according to disease indication/treatment protocol is warranted.

The effect of female tobacco smoking on IVF outcomes.

BACKGROUND: Cigarette smoking is widely believed to be associated with decreased fecundity in naturally conceiving populations; however, the effect of female smoking on pregnancy outcomes in patients undergoing IVF is unclear. METHODS: A retrospective analysis of 389 consecutive patients undergoing first cycle IVF was performed. Outcomes of peak estradiol (E(2)) levels, log mean ovarian volume, number of oocytes retrieved, oocyte maturity in ICSI, fertilization rate, cleavage rate, embryo quality, percentage of high-quality embryos, pregnancy and live birth were assessed in patients reported as never smokers, past smokers and current smokers. Potential confounding variables evaluated included day 3 FSH, number of oocytes retrieved, embryo quality, caffeine and alcohol consumption. The population was also stratified by female age (<35 and >or=35 years). RESULTS: A total of 9.3% of our patients reported current smoking and 12.1% reported a history of smoking. Smoking status did not significantly affect pregnancy outcome, live birth rate or any other indicated outcome. CONCLUSIONS: A total of 21.4% of IVF patients in this study had past or present exposure to cigarette smoking with no measurable effect on IVF outcome.

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.

Acute effects of bright light and caffeine on nighttime melatonin and temperature levels in women taking and not taking oral contraceptives.

Caffeine and bright light effects on nighttime melatonin and temperature levels in women were tested during the luteal phase of the menstrual cycle (n=30) or the pseudo luteal phase for oral contraceptive users (n=32). Participants were randomly assigned to receive either bright (5000 lux) or dim room light (<88 lux) between 20:00 and 08:00 h under a modified constant routine protocol. Half the subjects in each lighting condition were administered either caffeine (100 mg) or placebo in a double-blind manner at 20:00, 23:00, 02:00 and 05:00 h. Results showed that the combination of bright light and caffeine enhanced nighttime temperature levels to a greater extent than did either caffeine or bright light alone. Both of the latter groups had higher temperature levels relative to the dim light placebo condition and the two groups did not differ. Temperature levels in the bright light caffeine condition were maintained at near peak circadian levels the entire night in the luteal and pseudo luteal phase. Melatonin levels were reduced throughout the duration of bright light exposure for all women. Caffeine reduced the onset of melatonin levels for women in the luteal phase, but it had little effect on melatonin levels for oral contraceptive users. The results for women in the luteal phase of the menstrual cycle are consistent with our previous findings in men. The results also suggest that oral contraceptives may alter the effects of caffeine on nighttime melatonin levels.

Effects of menstrual cycle phase and oral contraceptives on alertness, cognitive performance, and circadian rhythms during sleep deprivation.

The influence of menstrual cycle phase and oral contraceptive use on neurobehavioral function and circadian rhythms were studied in healthy young women (n = 25) using a modified constant routine procedure during 24 h of sleep deprivation. Alertness and performance worsened across sleep deprivation and also varied with circadian phase. Entrained circadian rhythms of melatonin and body temperature were evident in women regardless of menstrual phase or oral contraceptive use. No significant difference in melatonin levels, duration, or phase was observed between women in the luteal and follicular phases, whereas oral contraceptives appeared to increase melatonin levels. Temperature levels were higher in the luteal phase and in oral contraceptive users compared to women in the follicular phase. Alertness on the maintenance of wakefulness test and some tests of cognitive performance were poorest for women in the follicular phase especially near the circadian trough of body temperature. These observations suggest that hormonal changes associated with the menstrual cycle and the use of oral contraceptives contribute to changes in nighttime waking neurobehavioral function and temperature level whereas these factors do not appear to affect circadian phase.

Combination of bright light and caffeine as a countermeasure for impaired alertness and performance during extended sleep deprivation.

Effects of four conditions (Dim Light-Placebo, Dim Light-Caffeine, Bright Light-Placebo and Bright Light-Caffeine) on alertness, and performance were studied during the night-time hours across 45.5 h of sleep deprivation. Caffeine (200 mg) was administered at 20.00 and 02.00 hours and bright-light exposure (> 2000 lux) was from 20.00 to 08.00 hours each night. The three treatment conditions, compared to the Dim Light-Placebo condition, enhanced night-time performance. Further, the combined treatment of caffeine and all-night bright light (Bright Light-Caffeine) enhanced performance to a larger degree than either the Dim Light-Caffeine or the Bright Light-Placebo condition. Beneficial effects of the treatments on performance were largest during the early morning hours (e.g. after 02.00 hours) when performance in the Dim Light-Placebo group was at its worst. Notably, the Bright Light-Caffeine condition was able to overcome the circadian drop in performance for most tasks measured. Both caffeine conditions improved objective alertness on the Maintenance of Wakefulness Test. Taken together, the above results suggest that the combined treatment of bright light and caffeine provides an effective intervention for enhancing alertness and performance during sleep loss.

Caffeine and light effects on nighttime melatonin and temperature levels in sleep-deprived humans.

The effects of caffeine ingestion and exposure to bright light, both separately and in combination, on salivary melatonin and tympanic temperature were assessed in humans. Four treatments during a 45.5 h sleep deprivation period were compared: Dim Light-Placebo, Dim Light-Caffeine, Bright Light-Placebo and Bright-Light Caffeine. The Dim Light-Caffeine condition (200 mg twice each night) relative to the Dim Light-Placebo condition suppressed nighttime melatonin levels and attenuated the normal decrease in temperature. Combining caffeine ingestion with bright light exposure (> or = 2000 lux) suppressed melatonin and attenuated the normal nighttime drop in temperature to a larger degree than either condition alone; i.e. effects were additive. Circadian effects were also observed in that the amplitude and phase of the temperature rhythm were altered during treatment. These findings establish that the human melatonin system is responsive to caffeine. Other evidence suggests that caffeine may influence melatonin and temperature levels through antagonism of the neuromodulator adenosine.

Topographical and temporal patterns of brain activity during the transition from wakefulness to sleep.

Changes in electroencephalographic (EEG) spectral power, coherence and frequency were examined for the last minute of wakefulness and the first minute of sleep via topographical mapping. Data were also analyzed across sequential 1-minute samples of wake, stage 1 and stage 2 sleep. Not all brain regions exhibited the same EEG changes during the transition and not all brain regions were found to change at the same time. Brain sites closest to the midline (e.g. F4, C4, P4, O2) showed significant changes in EEG power (increases in theta and decreases in alpha power) during the transition to sleep, whereas brain sites most lateral to the midline (e.g. Fp2, F8, T4) showed little change. Decreases in alpha coherence were observed from wakefulness to sleep for brain site comparisons furthest away from each other (e.g. T3 vs. T4, T5 vs. T6, F7 vs. F8, F3 vs. O1, F4 vs. O2). Spectral analysis of EEG activity revealed that the time of significant change in EEG power varies among brain regions. Decreases in alpha power continued to occur later into the transition period for the posterior regions of the brain (O2, P4).

EEG responses to low-level chemicals in normals and cacosmics.

Recent studies from the University of Arizona indicate that normal subjects, both college students and the elderly, can register the presence of low-intensity odors in the electroencephalogram (EEG) in the absence of conscious awareness of the odors. The experimental paradigm involves subjects sniffing pairs of bottles, one containing an odorant (e.g. isoamyl acetate) dissolved in an odorless solvent (water or liquid silicone), the other containing just the solvent, while 19 channels of EEG are continuously recorded. For the low-intensity odor conditions, concentrations are adjusted downward (decreased) until subjects correctly identify the odor bottle at chance (50%). The order of odorants, concentrations, and hand holding the control bottle, are counterbalanced within and across subjects. Three previous experiments found that alpha activity (8-12 hz) decreased in midline and posterior regions when subjects sniffed the low-intensity odors. The most recent study suggests that decreased theta activity (4-8 hz) may reflect sensory registration and decreased alpha activity may reflect perceptual registration. In a just completed experiment involving college students who were selected based on combinations of high and low scores on a scale measuring cacosmia (chemical odor intolerance) and high and low scores on a scale measuring depression, cacosmic subjects (independent of depression) showed greater decreases in low-frequency alpha (8-10 hz) and greater increases in low-frequency beta (12-16 hz) to the solvent propylene glycol compared to an empty bottle. Topographic EEG mapping to low-intensity odorants may provide a useful tool for investigating possible increased sensitivity to specific chemicals in chemically sensitive individuals.

Nonsteroidal anti-inflammatory drugs affect normal sleep patterns in humans.

Previous studies have demonstrated that some nonsteroidal anti-inflammatory drugs (NSAIDs), specifically aspirin and indomethacin, have acute negative effects on sleep in humans and animals. Whether this finding can be replicated and extended to other NSAIDs, particularly the widely used over-the-counter drugs ibuprofen and acetaminophen, was the focus of the present investigation. Thirty-seven male and female subjects slept in the sleep laboratory on 2 consecutive nights; sleep was polygraphically recorded on the second night. Three doses of a prostaglandin-inhibiting drug (i.e., aspirin, acetaminophen, or ibuprofen) or placebo were administered, one each at 2300 h on the day prior to sleep recording, and at 0815 h and 2300 h on the day sleep was recorded. Subjects slept from 2400-0800 h both nights. Aspirin and ibuprofen disrupted sleep in comparison to placebo by increasing the number of awakenings and percentage of time spent in stage wake, and by decreasing sleep efficiency. Ibuprofen also delayed the onset of the deeper stages of sleep. Acetaminophen did not differ significantly from placebo on any measure of polygraphically recorded sleep. However, every index of objective sleep reflected slight, albeit nonsignificant, sleep disruption for each drug group relative to placebo. The mechanisms of sleep disruption after NSAID administration may relate to direct and indirect consequences of inhibiting prostaglandin synthesis, including decreases in prostaglandin D2, suppression of nighttime melatonin levels, and changes in body temperature.

Developmental changes in memorial comparisons: the effects of stimulus presentation mode.

First graders, fifth graders, and college students made comparative size judgments of either pictures (line drawings) or names (spoken words) of common objects by designating the "bigger" item in real life. Care was taken to equate the picture and word conditions on a number of critical parameters including method of item-pair presentation and activation of response-time intervals. All groups exhibited a symbolic distance effect. While judgments were faster with pictures than words, the magnitude of the difference did not change with age. Previous research suggesting a marked developmental decline in the magnitude of the "pictorial superiority effect" may have confounded reduced memory demands with stimulus presentation mode for young children. Finally, slopes of the symbolic distance functions were found to decrease with increasing grade level, at least from first to fifth grade. This is the first demonstration of an age-related decline in slopes for magnitude comparisons of concrete objects.