Urinary 6-sulphatoxymelatonin Excretion Research Articles

Indoor cold exposure and nocturia: a cross-sectional analysis of the HEIJO-KYO study.

To investigate the association between indoor cold exposure and the prevalence of nocturia in an elderly population. The temperature in the living rooms and bedrooms of 1 065 home-dwelling elderly volunteers (aged ≥60 years) was measured for 48 h. Nocturia (≥2 voids per night) and nocturnal urine production were determined using a urination diary and nocturnal urine collection, respectively. The mean ± sd age of participants was 71.9 ± 7.1 years, and the prevalence of nocturia was 30.8%. A 1 °C decrease in daytime indoor temperature was associated with a higher odds ratio (OR) for nocturia (1.075, 95% confidence interval [CI] 1.026-1.126; P = 0.002), independently of outdoor temperature and other potential confounders such as basic characteristics (age, gender, body mass index, alcohol intake, smoking), comorbidities (diabetes, renal dysfunction), medications (calcium channel blocker, diuretics, sleeping pills), socio-economic status (education, household income), night-time dipping of ambulatory blood pressure, daytime physical activity, objectively measured sleep efficiency, and urinary 6-sulphatoxymelatonin excretion. The association remained significant after adjustment for nocturnal urine production rate (OR 1.095 [95% CI 1.042-1.150]; P < 0.001). Indoor cold exposure during the daytime was independently associated with nocturia among elderly participants. The explanation for this association may be cold-induced detrusor overactivity. The prevalence of nocturia could be reduced by modification of the indoor thermal environment.

The influence of indoor lighting with low blue light dose on urine 6-sulphatoxymelatonin concentrations and sleep efficiency of healthy volunteers

Light, especially its blue component, is the main synchronizer of circadian rhythms. We investigated effects of suppressed blue band of the spectrum on melatonin production and sleep efficiency in 18 young volunteers. During control days, participants lived in their home environment, and next five days in a room lit only by daylight with windows equipped with a filter blocking the blue band of the light spectrum. Light intensity, circadian stimulus and light irradiance were monitored. No significant changes in the daily pattern and total urinary 6-sulphatoxymelatonin excretion were found between control and experimental conditions. Parameters of sleep efficiency measured by wrist actigraphy were not worsened, but neutral chronotypes exhibited shortened sleep duration under light-modified conditions. We conclude that young healthy people can compensate for negative effects of transitory-worsened lighting conditions on their daily rhythms, but chronotypes and other personal characteristics may modify biological responses and should be considered.

Antioxidant defence and inflammatory response in professional road cyclists during a 4-day competition

Cycling competitions represent an important physical overload even for well-trained individuals. In six professional cyclists, we studied the adaptive oxidative and anti-inflammatory response to a 4-day road cycling competition and its relationship with melatonin, an antioxidant and anti-inflammatory stress hormone. Blood and urine samples were collected before and after the competition. Plasma lipid peroxidation, cytokines (interleukin-1β, interleukin-6, and tumour necrosis factor-α), creatine kinase and other metabolic markers, melatonin, erythrocyte glutathione, and glutathione peroxidase and reductase activities were measured. Urinary excretion of 6-sulphatoxymelatonin was analysed. Lipid peroxidation increased after the competition, but the erythrocyte glutathione pool remained unchanged. Changes in both glutathione peroxidase and reductase activities probably account for the recycling of glutathione after exercise. Interleukin-6 (216%) and tumour necrosis factor-α (159%) but not interleukin-1β increased after exercise. A parallel increase in plasma melatonin concentrations was detected, whereas metabolic markers, including creatine kinase, showed minor modifications. Thus, professional cyclists display an adaptative response to the physical overloads in the competitions for which they are trained. Consequently, they seem to be able to regulate efficiently the intracellular oxidative stress, and prevent an exaggerated pro-inflammatory cytokines induction. A modulator role of melatonin in these adaptive responses is also supported.

Daily Profiles of Salivary and Urinary Melatonin and Steroids in Healthy Prepubertal Boys

The aim of this study was to assess the circadian hormonal profile of two circadian markers, melatonin and cortisol, as well as other steroids in prepubertal boys (Tanner stage I). Nine volunteer healthy prepubertal boys aged 10.8 +/- 0.11 years participated in this study. Concentrations of daily salivary and urinary hormones were quantified around 24-hours, every 3 hours, in daytime samples (collected between 07.00 h +/- 30 min and 21.00 h +/- 30 min) and night-time samples (collected between 21.00 h +/- 30 min and 07.00 h +/- 30 min). Significant differences (p < 0.01) were found between day- and nighttime secretion of salivary melatonin and urinary 6-sulphatoxymelatonin, whereas no significant differences were found between day- and nighttime secretion of salivary and urinary cortisol nor between day- and nighttime secretion of 17-hydroxycorticosteroids (17-OHCS). The circadian profiles of salivary melatonin and cortisol showed large amplitude with a peak occurring at night (approximately 03.00 h) for melatonin and in the early morning (between 06.00 and 09.00 h) for cortisol. The curve patterns of the urinary 6-sulphatoxymelatonin and steroids (free cortisol and 17-OHCS) were coherent with data on saliva. The pattern of salivary androstenedione and testosterone were undetectable due to the very low concentrations of these steroids in the saliva of the prepubertal children. A strong significant positive correlation was observed between the daily salivary melatonin levels and the daily urinary 6-sulphatoxymelatonin excretion (R = 0.968, p < 0.001), and between free urinary cortisol and urinary 17-OHCS (R = 0.733, p = 0.025). The salivary and urinary hormones studied were independent of body mass index. This study shows the relevance of salivary cortisol and melatonin, although lower than in plasma, in testing adrenal and pineal function as markers of circadian rhythms. The data are of interest for the diagnosis and treatment of chronobiological disorders in prepubertal children.

One week of exposure to 50 Hz, vertical magnetic field does not reduce urinary 6-sulphatoxymelatonin excretion of male wistar rats.

The effect of exposure to 100 or 50 microT, 50 Hz, vertical magnetic field on the excretion of 6-sulphatoxymelatonin (6SM) in the nocturnal urine of rats was studied. Twelve male Wistar rats were kept under 12:12 hr light:dark conditions. The nocturnal urine of animals was collected in metabolic cages over 4 consecutive weeks. The concentration of 6SM in the rat urine was measured by 125I radioimmunoassay and normalized to creatinine concentration. After the first week of urine collection, 6 rats were exposed to 100 microT or 50 microT flux density magnetic fields (MF) for 8 hr daily for 1 week. It was found that the excretion of the primary metabolite of melatonin in the urine, 6SM, did not show statistically significant changes during and after magnetic field exposure.

Seasonal rhythms of 6-sulphatoxymelatonin (aMT6s) excretion in female rats are abolished by growth of malignant tumors.

The effect of development and growth of malignant tumors on pineal melatonin production was studied in two different hormone-dependent tumor systems in female rats. Urinary excretion of 6-sulphatoxymelatonin (aMT6s), the metabolic end product of melatonin, which parallels its production, was determined by radioimmunoassay at fortnightly or monthly intervals over the period of 1 year in female F344 Fischer rats bearing chemically-induced mammary carcinomas and in BDII/Han rats with spontaneous endometrial carcinomas. Untreated Fischer rats and BDII/Han rats in which tumor growth was suppressed by treatment with a progestin served as controls. Based on the cosinor analysis, animals without tumors showed significant seasonal rhythms of aMT6s excretion, with peaks in August (Fischer rats) and in May (BDII/Han rats). In contrast, such rhythms were absent in animals with developing and manifest tumors. It is concluded that animals kept under constant environmental conditions still show seasonal rhythms of pineal activity. Tumor development and growth affect pineal activity leading to disturbance of these rhythms.

The impact of fetal size and length of gestation on 6-sulphatoxymelatonin excretion in adult life.

Recent studies have shown that intrauterine growth retardation or fetal distress in human infants is associated with a pronounced reduction in melatonin secretion during the first 3 months of life. It is not known whether these associations persist beyond infancy. We have therefore examined the relationship between birthsize and melatonin secretion in 159 men and women aged 20, born in Adelaide, South Australia. Melatonin secretion was estimated by analysing the overnight urinary excretion of 6-sulphatoxymelatonin. The overnight excretion ranged from 1.7 to 128.9 nmoles/subject, was higher in women than in men (46.5 vs 34.1 nmoles, P = 0.003) and was significantly negatively correlated with the body mass index (P = 0.006). Excretion correlated with both birthweight and ponderal index at birth (P = 0.04 and P = 0.01 respectively after adjustment for gestational age) and also fell with increased duration of gestation (P = 0.007). The effects of adult body mass index added to that of low birthweight in predicting 6-sulphatoxymelatonin excretion. These data suggest that urinary 6-sulphatoxymelatonin excretion was impaired in adults who were growth restricted prenatally or were delivered after 40 weeks gestation.

Serotonin, excitatory amino acids and the photic control of melatonin rhythms and SCN c-FOS in the rat

There is a growing acceptance that serotonergic pathways to the suprachiasmatic nucleus play an important role in the mediation and modulation of light entrainment of rhythms. In this study administration of the 5-HT 2A/2C agonist (±)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI, 0.5 mg/kg) at mid dark caused a phase shift in the onset of the urinary excretion of 6-sulphatoxymelatonin in rats that was sustained for at least 8 days and was blocked by the specific 5-HT 2C antagonist SB-242084. Administration of DOI (2 mg/kg) across the night resulted in the appearance of c-FOS in the nucleus of cells in the suprachiasmatic nucleus during subjective darkness, but did not cause induction at the time of expected lights on (CT0). By contrast light exposure induced c-fos throughout the night including CT0. Administration of the NMDA receptor antagonist MK-801 (3 mg/kg) prior to light pulses had no effect on c-fos in the first part of the night, but towards the expected time of lights on, became progressively more potent, such that by CT0, light induction of c-fos was almost completely inhibited. These results provide further evidence that serotonin plays a role in the mediation of light effects on rhythms in the rat.

Low urinary 6-sulphatoxymelatonin levels in patients with coronary artery disease.

A decrease in nocturnal serum melatonin levels was reported in patients with clinically uncharacterized coronary artery disease. To assess whether there was a correlation between melatonin production and disease stage, we measured the nocturnal urinary excretion of 6-sulphatoxymelatonin (an index of blood melatonin concentration) in patients with chronic stable or unstable coronary disease and in a group of age-matched controls. Three groups of individuals were studied: a) 24 healthy subjects (mean age: 63 +/- 13 yr); b) 32 patients with chronic, stable, coronary disease (62 +/- 11 yr); and c) 27 patients with unstable angina (62 +/- 12 yr). For 6-sulphatoxymelatonin measurement, urine was collected from 18:00 to 06:00 hr, within 48 hr of hospitalization in the case of unstable angina. 6-Sulphatoxymelatonin was measured by a specific radioimmunoassay. Urinary 6-sulphatoxymelatonin excretion was significantly lower in unstable angina patients than in healthy subjects or in patients with stable angina. 6-Sulphatoxymelatonin correlated negatively with age in healthy subjects, but not in coronary patients. 6-Sulphatoxymelatonin excretion in patients treated with beta-adrenoceptor blockers did not differ significantly from coronary patients not receiving beta-blockers. The results indicate that patients with coronary disease have a low melatonin production rate, with greater decreases in those with higher risk of cardiac infarction and/or death.

Circadian rhythm abnormalities of melatonin and haploinsufficiency of COPS3 in Srnith-Magenis Syndrome

Smith-Magenis syndrome (SMS) is a multiple congenital anomalies, mental retardation syndrome associated with a hemizygous deletion of chromosome band 17p 11.2. The majority of patients have the same sized deletion; however, patients with smaller and larger deletions have been described. Characteristic features of SMS include a significant sleep disturbance. Urinary excretion of 6-sulphatoxymelatonin (aMT6s)—the major hepatic metabolite of melatonin—in conjunction with 24-hour sleep studies in 28 SMS patients reveal an aberrant diurnal rhythm of aMT6s and abnormalities in quality and quantity of nocturnal and daytime sleep. These data demonstrate a disturbed circadian rhythm of melatonin and objectively document the disturbed sleep pattern in Smith-Magenis syndrome. Our findings suggest that these abnormalities may be secondary to aberrations in the production, secretion, distribution, or metabolism of melatonin. One candidate, the gene for subunit 3 (COPS3) of the COP9 signalosome is conserved from plants to humans. In plants COP9 transduces light signals into gene expression, and in humans COPS3 maps within the SMS common deletion interval. We are investigating the hypothesis that COPS3 haploinsufficiency may be responsible for the abnormal melatonin circadian rhythm in SMS.

The role of melatonin in pathogenesis of aspirin-sensitive asthma.

Platelets are involved in the pathogenesis of bronchial asthma, including aspirin-sensitive asthma (ASA). The pineal hormone melatonin (MT) has been described as inhibiting several physiological processes in platelets. The MT metabolite - N-acetyl-5-methoxy-kynurenamine - has a chemical structure similar to that of acetylsalicylic acid. Because ASA patients usually suffer from an active disease, despite the avoidance of aspirin and cross-reactive drugs, it has been suggested that the MT synthesis may be lower and sensitivity of platelet reception to MT may be higher in aspirin-sensitive asthmatic subjects than in aspirin-tolerant ones. The objective of this study was to investigate this hypothesis. We studied the urinary excretion of 6-sulphatoxymelatonin, the major metabolite of melatonin, and the effect of different MT doses on ADP-induced platelet aggregation in vitro in 17 ASA patients, 17 patients with aspirin-tolerant asthma (ATA) and 16 healthy subjects. The results of the study have revealed a lower level of daytime aMT6s excretion in ASA patients (12.7 +/- 2.7 ng mL-1) than in ATA patients (32.7 +/- 9.9 ng mL-1, P < 0.05) and control subjects (19.9 +/- 5.8 ng mL-1, P < 0.05). The preincubation of platelet-rich plasma with the MT in a dose of 0.01 pg mL-1 plasma results in an increase in the intensity and the rate of the first platelet aggregation phase only in the ASA patients compared with the ATA patients and control subjects. We conclude that the reduction in MT production in ASA patients defines the character of ADP-induced platelet aggregation and the change in its first phase after the further addition of MT in vitro. The latter is associated with the opening of some receptor-operated channels for Ca2+ or/and its mobilization from the intracellular stores. The higher sensitivity of platelet reception and a distorted reaction not only to MT but also to its metabolite may be prerequisites for aspirin intolerance in ASA patients.

Urinary 6-Sulphatoxymelatonin Excretion of Rats is not Changed by 24 Hours of Exposure to A Horizontal 50-HZ, 100-μT Magnetic Field

The effect of exposure to a 50-Hz horizontal magnetic field on the excretion of urinary 6-sulphatoxymelatonin (aMT6s) into nocturnal urine of rats was studied. Two directions of the horizontal magnetic field, parallel and perpendicular to the magnetic north, were investigated in two separate experiments. Twenty-two male Wistar rats were kept in metabolic cages under 9:15 1ight:dark conditions for 5 days; the nocturnal urine was collected daily. The concentration of aMT6s in the rat urine was measured by 1251-radioimmunoassay. After the first 2 days, the rats were exposed to 100or 1-pT flux density magnetic fields for 24 h. On the fourth and fifth days, the urine collection was continued without exposure. It was found that the excretion of aMT6s, the primary urinary metabolite of melatonin, did not show statistically significant alteration during or after 50-Hz horizontal magnetic field exposure of rats to 100 or 1 pT flux density.

Total 24-hour melatonin secretion in adolescent idiopathic scoliosis. A case-control study.

A case-control study of 24-hour urinary melatonin production in patients with adolescent idiopathic scoliosis. To address the controversy over the role of melatonin deficiency in adolescent idiopathic scoliosis by measuring total melatonin production over a 24-hour period. An association between melatonin deficiency and experimental scoliosis has been suggested in several animal species. Recent work has failed to show a deficiency in humans with scoliosis. However, this conclusion was based on single urinary estimations. In this study the period assayed was standardized to 24-hours for all patients to include the full diurnal cycle of melatonin excretion. Consecutive patients at an outpatient clinic for adolescent idiopathic scoliosis were recruited as subjects for this study, and patients from a fracture clinic who were of similar age and gender were recruited as controls at their final follow-up examination after the healing of their fracture. Patients and control individuals collected urine over a 24-hour period that was divided into consecutive day and night collections of 12 hours each. Total urinary excretion of 6-sulphatoxy melatonin was determined by radioimmunoassay for each 12-hour period in patients and control individuals. No significant difference in diurnal, nocturnal, or total urine 6-sulphatoxy melatonin excretion was found between adolescent patients with idiopathic scoliosis and controls of similar age and gender. There was also no difference between the two groups when 6-sulphatoxy melatonin excretion was corrected for body weight, body surface area, and body mass index. Nor was there a significant difference between 6-sulphatoxy melatonin excretion of patients with scoliosis whose curves failed to progress over the course of a year and the excretion of those who underwent surgery. In adolescent idiopathic scoliosis, neither the presentation with a stable spinal deformity, nor presentation with a severe deformity requiring surgery is associated with melatonin deficiency.

Urinary 6-sulphatoxymelatonin excretion is increased in rats after 24 hours of exposure to vertical 50 Hz, 100 microT magnetic field.

The effect of exposure to a 50 Hz, vertical magnetic field on the excretion of urinary 6-sulphatoxymelatonin (aMT6s) of rats was studied in a self-controlled experiment. Ten male Wistar rats were kept under 9:15 h light: dark conditions in metabolic cages. The rats were exposed to 1.0 or 100 microT flux density for 24 h. The excretion of aMT6s, which is the primary metabolite of melatonin in the urine, did not show a statistically significant decrease, as measured by 125I radioimmunoassay, during or after magnetic field exposure of rats to either flux density. At 100 microT flux density, the increase of aMT6s excretion on the day after exposure was statistically significant (P < .02), compared with the value under exposure, but was not significant compared with the baseline values before exposure.

Sinusoidal 50 Hz, 500 microT magnetic field has no acute effect on urinary 6-sulphatoxymelatonin in Wistar rats.

The effect of a 50 Hz, vertical magnetic field on the excretion of urinary 6-sulphatoxymelatonin (aMT6s) of male and female Wistar rats was studied in a self-controlled experiment. Twenty rats were kept in metabolic cages under 9:15 h light:dark conditions. The urine of the animals was collected twice per day for 5 consecutive days. The concentration of aMT6s in the rat urine was measured by 125I radioimmunoassay. The rats were exposed to 5 and 500 microT flux density for 24 h. The excretion of urinary aMT6s did not show significant changes during or after magnetic field exposure.

Meditation, melatonin and breast/prostate cancer: Hypothesis and preliminary data

The objective of this study was to test the hypothesis that the regular practice of mindfulness meditation is associated with increased physiological levels of melatonin. Melatonin may be related to a variety of biologic functions important in maintaining health and preventing disease, including breast and prostate cancer. Previous studies have shown melatonin production is photosensitive and we suggest here that it also may be psychosensitive. A cross-sectional study of 12-hour (20:00 – 08:00) urinary 6-sulphatoxymelatonin was conducted from which we analyzed data from 8 women who regularly meditate (RM) and 8 women who do not meditate (NM). All samples were collected in the homes of study participants. Volunteers were recruited to provide 12-hour overnight samples of urine. All subjects collected the samples on one night during the same 1-week period. There was no explicit intervention. However, all RM were either graduates of, or teachers in, the University of Massachusetts Stress Reduction and Relaxation Program. The main outcome measure was the total excretion of urinary 6-sulphatoxymelatonin. Multiple linear regression (Proc GLM in SAS) was performed to test the effect of meditation (RM vs NM) on 6-sulphatoxymelatonin. The results of the study were that after controlling for the non-significant effect of menstrual period interval, we found an effect of meditation group (RM vs NM: b = 1.983; F = 6.78; p = 0.02) and age (for each integer year: b= −0.169; F = 8.41; p = 0.01). The conclusion is that study results are consistent with our hypothesis and indicate that melatonin might be a useful parameter in testing similar psycho-social interventions. Given that two intervention studies have provided support for the concept of psycho-physiological interactions in survival among cancer patients, applications of our findings might be pertinent to the area of breast and prostate cancer.

Acute and chronic idazoxan in normal volunteers: biochemical, physiological and psychological effects

The acute and chronic effects of the selective a(2)-antagonist idazoxan were studied in 12 normal volunteers. Plasma 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), blood pressure and psychological responses to oral challenge doses of idazoxan 40 mg were measured twice, on the first and 22nd day of treatment with idazoxan 40 mg t.d.s. Changes in nocturnal melatonin output were studied on six occasions, before, during and after idazoxan treatment. Although baseline MHPG levels were significantly reduced after chronic treatment with idazoxan, idazoxan challenge did not alter MHPG concentrations on either test day. A small rise in systolic blood pressure occurred after acute but not chronic idazoxan challenge tests. Systolic blood pressure values were significantly lower during the chronic compared with the acute test. Diastolic blood pressure and heart rate were not affected by acute or chronic treatment. Subjects reported increases in self- ratings of arousal and reductions in sedation and anxiety of similar magnitude after acute and chronic idazoxan. Nocturnal plasma melatonin secretion was not altered by drug administration or withdrawal, although urinary 6-sulphatoxymelatonin excretion was significantly reduced on acute withdrawal. The increase in systolic blood pressure and arousal self-ratings after acute idazoxan are in accordance with the reported effects of other a(2)-antagonists, although we did not find increased anxiety or elevated plasma MHPG levels. Chronic idazoxan appears to reduce or normalize activity of noradrenergic systems, indicated by reduced baseline systolic blood pressure and MHPG, and loss of the pressor response to idazoxan. Withdrawal of idazoxan leads to an abrupt fall in noradrenergic activity, as demonstrated by the fall in urinary 6-sulphatoxymelatonin.

The Effect of Sleep/Wake State on Nocturnal Melatonin Excretion

Twenty-four hour patterns of urinary 6-sulphatoxymelatonin excretion were monitored in eight healthy adult subjects in two bed rest constant routines, one with normal nocturnal sleep and one with continuous wakefulness. The implementation of dim light "constant routines" enabled the effect of the sleep wake/state on melatonin to be tested without the confounding effects of body activity and normal room lighting. In both conditions 6-sulphatoxymelatonin excretion was significantly higher during the nighttime hours (2200-1000) than during the daytime hours (1000-2200) producing averages of 80% and 78.5% of the total 24 hour output in the sleep and wakeful conditions, respectively. The large differences between subjects in nocturnal melatonin excretion (38 to 150 nmol) were highly consistent between the two conditions. There were no differences between the nocturnal wakeful and sleep conditions in total nighttime melatonin excretion nor in the nighttime percentage of the 24 hour total melatonin excretions. Therefore, the sleep/wake state alone had no effect on nocturnal melatonin excretion. On the other hand, a significant correlation between the nighttime melatonin percentage and sleep length suggested the need to investigate further the relation between the amplitude of the melatonin circadian rhythm and sleep length and quality.

Circadian rhythms in patients with abdominal pain syndromes.

Circadian rhythms for cortisol, 6-sulphatoxy melatonin and core body temperature were compared in control subjects and patients with functional abdominal pain. There were 20 patients with biliary pain after cholecystectomy, ten with biliary pain without cholecystectomy and 14 with irritable bowel syndrome. Rhythms were determined by urine collections at intervals of 4 h for 48 h and by overnight monitoring of core body temperature using the Vitalog system. Data were fitted to a sine curve to yield the time of maximal urinary excretion/lowest temperature and the amplitude of each rhythm. Urinary excretion of free cortisol was similar in patients and control subjects. For urinary 6-sulphatoxy melatonin, the timing (phase) of rhythms was similar in patients and controls but those with pain had a lower urinary excretion of 6-sulphatoxy melatonin (p less than 0.05) and a rhythm of lower amplitude (p less than 0.02). The amplitude of the temperature rhythm was also lower in patients with biliary pain with and without prior cholecystectomy (p less than 0.05). Functional abdominal pain is associated with 6-sulphatoxy melatonin and temperature rhythms of low amplitude, presumably because of suppression of circadian oscillators.

Annual changes in 6-sulphatoxymelatonin excretion in man.

A recently developed RIA for 6-sulphatoxymelatonin, the major urinary metabolite of melatonin, has been used to investigate the annual change in melatonin secretion in humans. Twenty plasma samples were taken from 18 volunteers throughout a 24-h period and simultaneous 6-hourly urine samples were also collected. Plasma melatonin and urinary 6-sulphatoxymelatonin were measured by RIA. 6-Sulphatoxymelatonin assayed in the urine samples was shown to be a good index of the rhythmic characteristics of the plasma melatonin secretion. To study annual changes in excretion four sequential 6-hourly urine samples were collected at monthly intervals from 16 normal volunteers for 13 months. Cosinor curves were fitted to the 6-sulphatoxymelatonin excretion data and the 24-h rhythm was described by the cosinor parameters: amplitude, mesor and acrophase. Significant differences in the acrophase were found during the year. The summer acrophase was phase advanced relative to the winter acrophase by about 1.5 h while intermediate phase positions were observed in spring/autumn. The 24-h excretion of urinary 6-sulphatoxymelatonin was remarkably consistent and there was no annual rhythm. In contrast, the daytime 6-sulphatoxymelatonin excretion between 12.00-18.00 h showed a statistically significant seasonal rhythm, with peaks in December/January and in July.