Persistent 24-h variations of urinary 6-hydroxy melatonin sulphate and cortisol in Antarctica

Biological rhythms during residence in polar regions

At Arctic and Antarctic latitudes, personnel are deprived of natural sunlight in winter and have continuous daylight in summer: light of sufficient intensity and suitable spectral composition is the main factor that maintains the 24-h period of human circadian rhythms. Thus, the status of the circadian system is of interest. Moreover, the relatively controlled artificial light conditions in winter are conducive to experimentation with different types of light treatment. The hormone melatonin and/or its metabolite 6-sulfatoxymelatonin (aMT6s) provide probably the best index of circadian (and seasonal) timing. A frequent observation has been a delay of the circadian system in winter. A skeleton photoperiod (2 × 1-h, bright white light, morning and evening) can restore summer timing. A single 1-h pulse of light in the morning may be sufficient. A few people desynchronize from the 24-h day (free-run) and show their intrinsic circadian period, usually >24 h. With regard to general health in polar regions, intermittent reports describe abnormalities in various physiological processes from the point of view of daily and seasonal rhythms, but positive health outcomes are also published. True winter depression (SAD) appears to be rare, although subsyndromal SAD is reported. Probably of most concern are the numerous reports of sleep problems. These have prompted investigations of the underlying mechanisms and treatment interventions. A delay of the circadian system with "normal" working hours implies sleep is attempted at a suboptimal phase. Decrements in sleep efficiency, latency, duration, and quality are also seen in winter. Increasing the intensity of ambient light exposure throughout the day advanced circadian phase and was associated with benefits for sleep: blue-enriched light was slightly more effective than standard white light. Effects on performance remain to be fully investigated. At 75°S, base personnel adapt the circadian system to night work within a week, in contrast to temperate zones where complete adaptation rarely occurs. A similar situation occurs on high-latitude North Sea oil installations, especially when working 18:00-06:00 h. Lack of conflicting light exposure (and "social obligations") is the probable explanation. Many have problems returning to day work, showing circadian desynchrony. Timed light treatment again has helped to restore normal phase/sleep in a small number of people. Postprandial response to meals is compromised during periods of desynchrony with evidence of insulin resistance and elevated triglycerides, risk factors for heart disease. Only small numbers of subjects have been studied intensively in polar regions; however, these observations suggest that suboptimal light conditions are deleterious to health. They apply equally to people living in temperate zones with insufficient light exposure.

Saliva cortisol levels in construction workers in the Arctic (78°N)

OBJECTIVES

The aim was to investigate how working in an extreme and isolated environment in the Arctic affected the diurnal rhythm of saliva cortisol.

STUDY DESIGN

Field study.

METHODS

Twenty-five male tunnel workers were screened during 3 different working cycles with different light conditions during a 9-month construction period; April/May (24 hours [h] light), September/October (approximately 12 h light and 12 h darkness) and November/December (24 h darkness). The work schedule was 10 h on/14 h off, 21 days at work/21 days off work. The workers alternated between the day shift in 1 work period and the night shift in the next. Four saliva samples were collected on day 14 in all 3 periods; immediately after awakening, and then 30 minutes, 6 hours and 12 hours after awakening.

RESULTS

Regardless of shift schedule, the workers' cortisol levels were significantly lower in the period with 24 hours of light per day compared to the period with "normal" light conditions. There were no differences in the cortisol levels of the workers on night shifts in the period with 24 hours of darkness compared to those in the period with "normal" light conditions, but the workers who were on day shifts in the period with 24 of hours darkness had a disturbed cortisol rhythm (lower peak after awakening and lack of the normal decrease during the day).

CONCLUSIONS

External light conditions and shift schedule were important factors in regulating the workers' cortisol rhythm. It seems to be easier to adapt to a night rhythm than an early morning rhythm in an isolated and extreme environment.

Seasonal variation in serum concentrations of reproductive hormones and urinary excretion of 6-sulfatoxymelatonin in men living north and south of the Arctic Circle: a longitudinal study

OBJECTIVE

Seasonal variation in photoperiod or temperature may influence human reproductive biology. The present study evaluated whether seasonal changes occurred in the levels of reproductive hormones and the major melatonin metabolite, 6-sulfatoxymelatonin (aMT6s), in populations exposed to extreme variation in photoperiod and temperature.

DESIGN

Two separate cohorts of Norwegian men were recruited from the general population in either of two locations: Tromsø (69.5 degrees N, n = 92) or Oslo (60 degrees N, n = 112), located north and south of the Arctic Circle (66.5 degrees N), respectively.

MEASUREMENTS

Four blood and 12-h overnight urine samples were obtained on separate occasions over a 12-month period, including during the photoperiod maximum and minimum. Serum concentrations of FSH, LH, testosterone (T), oestradiol (E(2)), SHBG and the urinary excretion of aMT6s were assessed.

RESULTS

Statistical analysis using generalized estimating equations indicated that LH levels were lowest during early winter in both locations (both P = 0.01). In Tromsø, free T and E(2) concentrations peaked during early winter (P = 0.02 and 0.003, respectively). In Oslo, free T levels were lowest during early winter (P = 0.06) whereas E(2) levels were lowest during late summer (P < 0.001). Urinary aMT6s concentrations were lowest during early summer in Tromsø and Oslo. Concentrations peaked during early winter in Tromsø (P < 0.001) and during late winter in Oslo (P < 0.001).

CONCLUSIONS

LH levels exhibited similar changes in both locations, whereas the patterns of changes of the sex steroid concentrations differed, possibly indicating different underlying mechanisms. Excretion of aMT6s was lowest during early summer in both locations, indicating that the long natural photoperiod was sufficient to cause suppression of melatonin secretion. Whether these changes have any biological significance remains uncertain.

Seasonal changes of human circadian rhythms in Antarctica

The human circadian rhythms in sleep, activity, plasma melatonin, and rectal temperature were explored under two conflicting time cues in Antarctica: an extreme photoperiod and a strict work schedule. The nine healthy male subjects stayed at the Antarctic zone (latitude 66.5-90 degrees south) for 15 mo including a 13-mo wintering at the Dome station (latitude 77 degrees south). Neither the phases nor the amounts of sleep and daily activity underwent a seasonal change. On the other hand, the peak phase of melatonin rhythm was phase delayed by 4.1 h in winter compared with summer. When the analysis is limited to the Dome data, the seasonal difference was reduced to 1.3 h. Similarly the trough phase of rectal temperature rhythm in two of three subjects was phase delayed by approximately 2 h in winter. From these findings, the sleep or activity rhythm is concluded to be reset predominantly by the work schedule, whereas the circadian rhythm in plasma melatonin and rectal temperature is substantially influenced by the photoperiod.

Daily and seasonal variations in the concentration of melatonin in the human pineal gland

To elucidate whether pineal melatonin secretion is affected by changes in day length, we determined the concentration of melatonin in human pineal glands obtained at autopsy from 66 male subjects, aged 16-84 years over a period of 12 consecutive months. Based on the time of death, a day-night difference in pineal melatonin levels was evident only in the long photoperiod (April-September) with significantly higher melatonin concentrations occurring at night (2200-1000 h). Nighttime values in the long photoperiod were significantly higher than the nighttime values during the short photoperiod (October-March). During the short photoperiod, the data suggested a possible phase-delay in melatonin secretion. Day-night difference was evident in young subjects (30-60 years), but not in elderly subjects (61-84 years). Elderly subjects had lower total melatonin levels (day and night values) although statistically not significant. Therefore, melatonin levels did not decline with age and when the data were analyzed by age there was no significant day-night difference in melatonin levels. These data indicate that the concentration of melatonin in the human pineal is augmented only during the long photoperiod. The results suggest a partial effect of photoperiod on melatonin secretion in humans. This may result from living in an artificial light environment or due to other nonphotic signals involved in generating melatonin rhythm.

Cortisol in light treatment of seasonal and non-seasonal depression: relationship between melatonin and cortisol

The effect of bright light on cortisol and the relationship between melatonin and cortisol were studied in 63 depressed patients (42 patients with a seasonal pattern and 21 patients with a non-seasonal pattern). The patients were matched for age, time of treatment and severity of depression. Before and after light treatment the severity of the depression was rated with the Comprehensive Psychopathological Rating Scale (23 items) and the Hamilton Depression Rating scale (18 items), and serum cortisol and melatonin were drawn at nine time-points between 20.00 and 08.00 hours. Two hours of light treatment (350 cd m-2) was given daily for 10 days either in the morning (06.00-08.00 hours) or in the evening (18.00-20.00 hours). As reported earlier, patients with a seasonal pattern improved significantly more than patients with a non-seasonal pattern of depression, and no significant differences were found between the treatment efficacy of morning compared to evening light. A cosinor analysis showed that the cortisol batyphase was significantly advanced by morning light, but was not delayed by evening light. A delay in batyphase cortisol showed a weak significant correlation with a decrease in the absolute and relative sum of scores. The batyphase of cortisol occurred approximately 3 h earlier than the acrophase of melatonin. Of the changes in the melatonin acrophase 43% were reflected in a change of cortisol batyphase, indicating a hierarchical relationship with melatonin as the co-ordinating hormone transducing part of the information of the external light to the phase position of cortisol. No significant differences between patients with a seasonal or a non-seasonal pattern were seen in mesor, amplitude or batyphase of cortisol before treatment, and no significant changes in mesor or amplitude were seen as a result of light treatment.

Urinary 6-sulfatoxymelatonin cycle-to-cycle variability

For either clinical or research purposes, the timing of the nocturnal onset in production of the urinary melatonin metabolite 6-sulfatoxymelatonin (UaMT6s-onset), has been proposed as a reliable and robust marker of circadian phase. However, given that most circadian rhythms show cycle-to-cycle variability, the statistical reliability of phase estimates obtained from a single study using UaMT6s-onset remains to be determined. Following 2 weeks of sleep diary and wrist actigraphy, 15 young, healthy good sleepers participated in four UaMT6s sampling sessions spaced 1 day apart. During the sampling sessions subjects remained indoors under low light conditions and hourly urine samples were collected from 19:00 to 02:00 h. Samples were subsequently assayed for UaMT6s using standard radioimmunographic techniques. UaMT6s-onset was determined by the time at which melatonin production exceeded the average of three proceeding trials by 100%. Sleep onset times were derived from sleep diary and actigraphic measures taken before the melatonin collection nights. We found that there was no significant variation between nights in group mean UaMT6s-onset times, and intraindividual variability was small. In addition, UaMT6s-onset times were highly and significantly correlated between nights (grand mean r = 0.804). Our results suggest that within 95% confidence interval limits, individual UaMT6s-onset estimates obtained from a single night UaMT6s-onset study can be used to predict subsequent UaMT6s-onset times within +/- 97 min. A close temporal relationship was also found between the timing of UaMT6s-onset and sleep onset. Overall, our results suggest that under entrained conditions single-session UaMT6s-onset studies can provide reliable individual UaMT6s-onset phase estimates and that the protocol described in this study is a practical and noninvasive methodology.

Nocturnal secretory patterns of melatonin, luteinizing hormone, prolactin and cortisol in male patients with gonadotropin-releasing hormone deficiency

To clarify whether disorders of gonadotropin releasing hormone (GnRH) deficiency are associated with altered melatonin and pituitary hormones secretory patterns, we studied male patients with hypogonadotropic hypogonadism (IGD; n = 6), delayed puberty (DP; n = 7) and age-matched pubertal controls (n = 7). Serum samples for the determination of melatonin, luteinizing hormone (LH), prolactin and cortisol levels were obtained at 15 min intervals from 1900 to 0700 in a controlled light-dark environment, complete bed-rest and fasting with simultaneous sleep recordings. Mean (+/- SD) dark-time melatonin levels were significantly higher in IGD (286 +/- 26 pmol/L) and DP (205 +/- 44 pmol/L) compared with 178 +/- 64 pmol/L in controls (P < 0.003). So were the mean (+/- SD) peak melatonin levels (453 +/- 63, 346 +/- 106 and 292 +/- 96 pmol/L) in IGD, DP and controls, respectively (P < 0.03). Integrated nocturnal melatonin (AUC) values were also higher in IGD and DP (184 +/- 15 and 134 +/- 28 pmol/min/L x 10(3)) compared with 116 +/- 42 pmol/min/L x 10(3) in controls (P < 0.003). The time of onset of the nocturnal melatonin rise was observed earlier in IGD and DP patients as compared to controls. No correlations were found between melatonin and LH levels, between melatonin and prolactin levels, or between melatonin and cortisol levels. These data indicate that melatonin secretion is enhanced in male patients with GnRH deficiency. The lack of correlation between melatonin and LH suggest that circulating gonadal steroids, rather than LH, modulate melatonin secretion in a reverse fashion.

MidnightSun: software for determining light exposure and phase-shifting schedules during global travel

The application of circadian principles has the potential to alleviate jet-lag in global travelers, but their application is hampered by the difficulty of determining light exposure along international flight routes. Computerized tools can solve this problem algorithmically. We have developed a program for Macintosh computers, called MidnightSun, which allows researchers to display ambient lighting conditions at any geographical location at any time of the year. The program contains a data base with the latitudes and longitudes of over 3000 airports. It calculates flight paths and durations, and prints a graphical itinerary indicating times of daylight during flights and layovers. Given a travel itinerary and a user-defined phase response curve (PRC) for light, it recommends light exposure times that may accelerate the reentrainment of circadian rhythms to new time zones and reduce the deleterious effects of jet-lag (depending on the efficacy of the PRC and the compliance of the traveler). Other potential applications include determining lighting protocols for photoperiodism experiments and providing data sets for mathematical circadian simulations under naturalistic lighting conditions.

Melatonin rhythms in Arctic urban residents

The 24-hr rhythm of salivary melatonin was measured in persons living in the city of Tromsø (70 degrees N) at the following times of the year: in January at a day length of 2 hr of twilight, in June under continuous sunshine, and in March and September at about 12 hr light and 12 hr darkness. The hormone patterns varied widely between individuals, but, in general, they were consistent within most individuals between the seasons. Highest peak values occurred in January when the mean level was also significantly higher than at any other time of year. The lowest mean levels occurred in June. Although individual rhythms were not always apparent, the mean patterns showed significantly elevated melatonin concentrations during the night at all seasons. The June melatonin peak was similar to that in March and September, but appeared to be phase-delayed with increased melatonin concentrations from midnight until 0900. It is assumed that the delayed melatonin peak in June may be associated with a tendency among people to shift their activity/rest rhythm and that the pineal sensitivity to light is reduced in the morning in summer.

Biological clocks: mechanisms and developments

Almost all organisms ranging from unicellular protists to mammals were found to show biological rhythms. Many workers have performed various kinds of experiment to understand the mechanism as well as to find the origin of the clock responsible for these rhythms. However, there is no doubt about the existence of a biologically controlled clock in almost all organisms; yet its origin and mechanism still remain a mystery. Many theories have been put forward to explain the mechanism of these biological clocks and it seems that the cell membrane may play a key role. The existence of a very high electric field of the order of 10(5) V cm-1 across the cell membrane may have some role in the mechanism of the biological clock. Of all the factors which have the effects on biological rhythms, light and temperature are found to be the most common. Also, the study of these biological clocks can help to solve the sleeping problems of international travellers and shift workers as well as to improve diagnosis, cure and prevention from diseases.

Variations in resting metabolic rates of men in Antarctica

The resting metabolic rates (RMR) of 6 men was determined monthly for 12 consecutive months, at Rothera Base, Antarctica (67 degrees 34'S, 68 degrees 07'W). Body weight and body composition were also recorded. Metabolic rates were within the range of those found in past polar studies, and of young men in the UK, but varied considerably from month to month. The RMR had a mean range of 30% (p less than 0.01), with individual ranges of up to 38%. Mean RMR was high in spring, summer and autumn, and low in winter. While these patterns appeared to be associated with a well defined annual cycle of activity, variations could not be correlated with periods of intense activity, individually or for the group.

Human melatonin production decreases with age

The purpose of this study was to investigate the effects of time of year and demographic variables on the amplitude of melatonin production in normal human subjects. Melatonin production was estimated by measuring the overnight excretion of its major urinary metabolite, 6-hydroxymelatonin. Urine was collected on three consecutive nights in the summer from a sample of 60 normal subjects balanced for sex and age. The collections were repeated in a subgroup during the winter. Melatonin production clearly declined with age but was not influenced by other demographic variables or by season of the year.